WASHINGTON — A US drone struck and killed at least 18 members of an allied Syrian force this week, the Pentagon said Thursday, in the worst friendly-fire incident of the war there against the Islamic State.
The strike Tuesday south of Tabquah, a strategic town in northern Syria, deepens questions about targeting methods used in the ongoing American air campaign over Iraq and Syria, which activists allege has resulted in a surge in civilian deaths this year.
The US-led coalition said this week’s incident occurred after Syrian forces erroneously identified another allied unit as a group of Islamic State fighters.
Separately, US forces in Afghanistan dropped a 21,000-pound bomb on Islamic State forces in eastern Afghanistan on Thursday, the Pentagon announced, using the largest non-nuclear bomb ever employed in combat.
General John Nicholson, the commander of US forces in Afghanistan, said the bomb was ‘‘the right munition’’ to use against the Islamic State because of the group’s use of roadside bombs, bunkers, and tunnels.
The bomb, which is known as the GBU-43, is one of the largest airdropped munitions in the US military’s inventory and was almost used during the opening salvos of the Iraq War in 2003.
By comparison, US aircraft commonly drop bombs that weigh 250 to 2,000 pounds.
The US military has targeted similar complexes and dropped tens of thousands of bombs in Afghanistan, raising the question of why a bomb of this size was used Thursday. It was unclear what the GBU-43 strike accomplished, as the bomb is not designed to penetrate hardened targets such as bunkers or cave complexes.
The Pentagon said in its statement that ‘‘US forces took every precaution to avoid civilian casualties with this strike.’’
President Trump has bestowed additional authority on the Pentagon in his first months in office, which the military has argued will help it defeat the Islamic State more speedily. Trump did not say whether he had personally approved Thursday’s mission.
“What I do is I authorize my military,” Trump said after a meeting with emergency workers at the White House. He called the bombing “another very, very successful mission.”
The Afghan bombing and the friendly-fire incident in Syria comes in the week after the Trump administration, promising a tough stance on a range of foreign-policy issues, launched a barrage of ship-borne cruise missiles against a Syrian government air base in retaliation for a deadly chemical attack on Syrian civilians.
The assault against a Syrian military facility, the first direct US attack on a Syrian government target since that country’s civil war began in 2011, appeared to be a momentary deviation from the campaign the United States and its Syrian allies are waging to defeat the Islamic State.
After the drone strike in northeast Syria this week, the Syrian Democratic Forces (SDF), an opposition group the United States is backing, said its fighters had fallen as ‘‘martyrs’’ during operations around Tabquah.
‘‘The general leadership of SDF in coordination with [the] international coalition will investigate the reasons behind the accident in order to prevent it happening again,’’ the SDF General Command said.
The SDF, a Kurdish-dominated force, has proved to be a key partner for the United States in Syria.
According to a US official with knowledge of the accidental strike, it was proceeded by a series of missteps.
First, an SDF unit operating close to Islamic State lines incorrectly reported its own location to the US-led coalition, the official said. Typically, friendly forces share their locations with the United States to keep themselves safe from air power.
Then, a separate SDF unit, which spotted the first unit from afar, mistakenly reported its fellow SDF fighters to be Islamic State elements and requested an airstrike on their location.
Armed with what American officials believed were coordinates for a legitimate target, the drone then conducted the attack, with deadly results.
The strike occurred at night, and the SDF units did not have night-vision gear, the US official said.
In an e-mail, the official said the US-led coalition had a strong track record of supporting SDF operations with air power.
‘‘The coalition is in close contact with our [Syrian] partners who have expressed a strong desire to remain focused on the fight against ISIS despite this tragic incident,’’ the coalition said in a statement.
ISIS is another name for the Islamic State, which the United States and its allies have been battling since 2014.
The situation around Tabquah is particularly complex, as Islamic State militants stage fierce battles in defense of positions near their de facto capital, Raqqa.
US officials say militants have used vehicles fitted with explosives, artillery, and mortar attacks, and have employed human shields in their efforts to slow forces advancing on Raqqa.
As part of the operation, US Special Operations forces and SDF troops landed behind Islamic State lines in the area last month, aiming to capture Tabquah as well as its strategic dam and airfield.
‘‘This was a very dynamic situation,’’ said the US official who commented on the accidental airstrike. ‘‘The ground units and coalition forces involved in this strike are well experienced and communicate often. Unfortunately, this dynamic situation resulted in loss of detailed location understanding.’’
It is not clear whether the incident involved Kurdish SDF forces, members of a smaller Arab component, or both. The US partnership with the SDF has caused intense friction with NATO ally Turkey, which sees the Kurdish fighters as a threat to Turkish security and has pushed the United States to instead back Turkish-selected units.
The rise of Kurdish militias in Syria has also created friction with Syrian Arabs, some of whom resent the Kurds’ attempts to consolidate territory under their control. The United States is planning to employ a smaller Arab component of the SDF to spearhead the assault on Raqqa, an Arab city.
The Tabquah strike follows two other friendly-fire incidents in the past month in the campaign against the Islamic State. Bombing runs killed or wounded scores of civilians in a mosque complex in Syria and in a building in the west neighborhoods of Mosul, Iraq.
The number of civilian casualties reported in US-led strikes has increased since Trump took office, and March was the deadliest month for civilians ever recorded there by Airwars, a group that tracks bombings. Reports of civilian casualties in Iraq and Syria jumped to 3,471 from 1,782 the month before, the group said.
US officials argue that the incidents are a regrettable but unavoidable outcome in a war that has included about 20,000 US and coalition airstrikes.
But the incidents illustrate the difficulties inherent in the US strategy in Iraq and Syria, which relies on local ground forces to do most of the fighting and, often, to provide information that is used in complex air operations.
Those challenges are made greater by the small scale of the US presence in both countries, meaning fewer American personnel on hand to verify targets firsthand. There are roughly 500 US Special Operations troops working with the SDF throughout Syria and approximately 1,000 US troops overall.
As part of an elaborate process for developing and approving targets for aerial attack, US officials say they use drone surveillance and other methods to cross-check information provided by ground forces, who often are requesting air power for their own defense in fast-moving situations.
But air missions are challenging in a crowded battlefield, and requests from local forces have played a role in some incidents of civilian casualties.
Those include the attack last month in Mosul.
Pentagon officials say the principal cause in the rise in civilian casualties is that the fight is entering a new, more intense phase. But activists have wondered whether changes to the military’s approvals process for strikes have contributed.
