BEIRUT — Syrians are being killed at an average rate of 5,000 per month, the U.N. said Thursday as it raised the overall death toll in the civil war to nearly 93,000, with civilians bearing the brunt of the attacks.
The grim estimate reflects the growing sectarian nature of the bloodshed as the regime of President Bashar Assad scores a series of battlefield successes against the rebels.
The toll also is a reminder of the international community’s helplessness in the face of a conflict that has displaced several million people and spilled over into neighboring countries with alarming frequency.
Adding to the pressure for international action, the United States announced Thursday it had conclusive evidence that Assad’s regime has used chemical weapons, including the nerve agent sarin, on a small scale against opposition forces. The White House said multiple chemical attacks last year killed up to 150 people.
President Barack Obama has said the use of chemical weapons by the Syria regime would be a ‘‘red line’’ that would trigger greater U.S involvement in the crisis. Officials said Obama was considering both political and military options, but it was unclear how quickly new actions would be taken and what they would involve.
In the latest violence, a mortar round slammed into an area near the runway at Damascus International Airport, briefly disrupting flights to and from the Syrian capital, officials said. The attack Thursday came a few weeks after the government announced it had secured the airport road that had been targeted by rebels in the past.
It was the first known attack to hit inside the airport, located south of the capital, and highlighted the difficulty Assad faces in maintaining security even in areas firmly under his control.
In Geneva, the U.N. human rights office said it had documented 92,901 killings in Syria between March 2011 and the end of April 2013. But the U.N. high commissioner for human rights, Navi Pillay, said it was impossible to provide an exact number, which could be far higher.
The figure was up from nearly 60,000 through the end of November, recorded in an analysis released in January. Since then, U.N. officials had estimated higher numbers, most recently 80,000. The latest report adds more confirmed killings to the previous time period and an additional 27,000 between December and April.
The conflict began in 2011 as largely peaceful protests of Assad’s autocratic regime. After a relentless government crackdown on the demonstrators, many Syrians took up arms against the regime, and the uprising became a civil war.
The government has gained momentum since recapturing the strategic town of Qusair near the border with Lebanon. Regime forces now appear set on securing control of the central provinces of Homs and Hama, a linchpin area linking Damascus with regime strongholds on the Mediterranean coast, and Aleppo to the north.
Most of the armed rebels in Syria are from the country’s Sunni majority, while Assad has retained core support among the minorities, including his own Alawite sect, an offshoot of Shiite Islam.
The international community has been unable to end the violence and a date for a peace conference proposed by the U.S. and Russia weeks ago has been shelved.
U.S. officials had hoped this week to reach a decision on arming Syria’s rebels but they are still uncertain whether that’s the best way to reshape a war that now includes Lebanon’s Hezbollah and Iranian fighters supporting Assad’s armed forces, and al-Qaida-linked extremists backing the opposition.
Obama and his national security team are ‘‘greatly concerned’’ by the worsening situation in Syria, White House spokesman Jay Carney said, speaking hours before the White House announced there was evidence that the Assad regime had used chemical weapons.
Carney said he expects Syria will be discussed at the Group of Eight summit next week in Northern Ireland.
The administration’s caution persists despite its nearly two-year demand that Assad step down, its vows to help the besieged Syrian rebels and its threats to respond to any chemical weapons use.
State Department spokeswoman Jen Psaki said the U.S. was saddened at the latest U.N. estimate, ‘‘but we’re not shocked.’’
She said Assad ‘‘has used indiscriminate and disproportionate force against Syrian civilians and inflicted unthinkable suffering upon his own people.’’
The Britain-based Syrian Observatory for Human Rights, which has its own count based on a network of activists around the country, said 98,416 people have been killed from the start of the uprising until Wednesday, many of them members of the military or pro-regime militiamen.
The Observatory’s figures include 36,139 civilians and 42,147 regime fighters, including military and defense forces, as well as pro-government militias known as the Popular Committees, shabiha and National Defense Forces.
‘‘The real figure could be higher than 120,000 or 130,000,’’ said Rami Abdul-Rahman, the head of the Observatory, citing regime secrecy on its losses and the difficulties for human rights group to work inside Syria.
The soaring death toll underscores the brutal nature of the conflict in Syria, even in comparison with sectarian conflicts that ravaged neighbors Lebanon and Iraq.
A review by the Associated Press in April 2009 showed that more than 110,600 Iraqis died in violence since the U.S.-led invasion six years earlier. The actual number was likely higher because many of those listed as missing were doubtless buried in the chaos of war without official records.
About 150,000 people are believed to have died in Lebanon’s 15-year civil war, which ended in 1990, and some 17,000 people are still missing. Sectarian sensitivities still prevail in Lebanon, more than two decades after the war ended.
The U.N. figures released Thursday in Geneva trace the arc of violence, with the average monthly number of documented killings rising from around 1,000 per month in summer 2011 to an average of more than 5,000 per month since July 2012. At its height from July to October 2012, the number of killings rose above 6,000 per month.
‘‘The constant flow of killings continues at shockingly high levels,’’ Pillay said. ‘‘This is most likely a minimum casualty figure. The true number of those killed is potentially much higher.’’
Among the victims were at least 6,561 children, including 1,729 younger than 10, the U.N. said.
‘‘There are also well-documented cases of individual children being tortured and executed, and entire families including babies being massacred — which, along with this devastatingly high death toll, is a terrible reminder of just how vicious this conflict has become,’’ Pillay said.
‘‘Civilians are bearing the brunt of widespread, violent and often indiscriminate attacks, which are devastating whole swaths of major towns and cities, as well as outlying villages,’’ Pillay said.
‘‘Government forces are shelling and launching aerial attacks on urban areas day in and day out,’’ she said. ‘‘Opposition forces have also shelled residential areas, albeit using less firepower, and there have been multiple bombings resulting in casualties in the heart of cities, especially Damascus.’’
Syrian Transportation Minister Mahmoud Ibrahim Said told Syrian TV that a mortar round fired by ‘‘terrorists’’ struck near a warehouse, breaking its windows and wounding a worker.
He said the attack delayed the landing of two incoming flights, from Latakia and Kuwait, as well as a flight to Baghdad. No passengers were harmed and no planes were damaged, he said. The regime commonly refers to rebels as ‘‘terrorists.’’
Tarek Wahibi, head of operations at the airport, said arrivals and departures later resumed to normal.
Rebels also battled regime forces for control of a key military base in the central province of Hama after chasing soldiers out and setting fire to installations there, activists said.
Following dawn battles, rebels took control of the base on the northern edge of the town of Morek, which straddles the strategic north-south highway leading to Aleppo.
By midday, regime forces shelled the base and sent reinforcements in an apparent attempt to regain control of the area, the Observatory said.
It added that rebels killed six government fighters and seized ammunition and weapons. Two rebel fighters were killed.
An amateur video posted on Hama activists’ Facebook page showed flames rising from the burning compound and the bodies of some of the fighters. In the video, fighters celebrated the fall of the base, calling it one of the ‘‘most critical’’ regime outposts in the region.
Also Thursday, the Observatory said troops shelled the eastern village of Hatla that damaged several homes and set others on fire. The attack came two days after rebels, including Sunni extremists, stormed the village and battled pro-regime militiamen, killing more than 60 Shiite fighters and civilians in an attack steeped in the sectarian hatred.
State TV reported that troops ambushed a group of rebels who were withdrawing from an area near Qusair, killing dozens of them. The TV said troops captured large amounts of weapons from the rebels.