BEIRUT — An explosion targeting Syrian regime forces killed at least three people in northern Syria on Wednesday — and possibly up to 18 — amid rising violence ahead of a visit by the new UN-Arab League envoy who is trying to end the country’s civil war.

Diplomatic efforts have so far failed to halt the bloodshed in Syria, but the new international envoy, Lakhdar Brahimi, will be in Syria on Thursday for talks with Foreign Minister Walid Moallem, according to ministry spokesman Jihad Makdissi.

UN Secretary General Ban Ki-moon said Brahimi also will meet with President Bashar ­Assad during the trip.

There were conflicting accounts of the nature of Wednesday’s blast and the number of casualties.

The Britain-based Syrian Observatory for Human Rights said that a car bomb exploded, and that 18 security agents were killed.

The Observatory said dozens of rebels attacked the post following the blast in the Idlib Province town of Saraqeb.

But a government official said a suicide attacker blew himself up in Saraqeb, killing two policemen and a civilian. The official spoke on condition of anonymity because he was not authorized to speak to the media.

An activist, who spoke on condition of anonymity out of fear of reprisals, also said the blast was carried out by a suicide bomber driving a car.

Syria restricts media access, making it difficult to independently verify such accounts.

Al Qaeda-style suicide bombings have become increasingly common in Syria, and Western officials say there is little doubt that Islamist extremists, some associated with the terror network, have made inroads in Syria as violence has engulfed the country. But the main fighting force looking to oust Assad is the Free Syrian Army, a group made up largely of defected Syrian soldiers.

The attack came after a blast Sunday in Aleppo destroyed parts of two hospitals where soldiers are usually treated. State media said 30 civilians were killed and 64 wounded in the Aleppo bombing.

Elsewhere, activists reported heavy fighting between government troops and rebels in Aleppo, much of it near the government-held Aleppo International Airport. Activist Mohammed al-Hassan said the airport, which includes a military base, is used by the regime to bomb rebel-held areas in Aleppo.

The state-run news agency, SANA, said troops repelled an attempt Wednesday by rebels to enter Aleppo and killed and wounded ‘‘a large number of them.’’

The Observatory and the Local Coordination Committees said at least 11 bodies were found in Halfaya in Hama Province. They said the bodies were found in fields a day after government troops stormed the town.

To the east, the group said, Syrian warplanes attacked the village of Souseh near the Iraq border, killing at least nine.

Activists say that more than 23,000 people have been killed since Syria’s crisis began in March last year.

The violence has force nearly 300,000 Syrians to flee the country, and the vast majority of them have sought refuge in neighboring Jordan, Turkey and Lebanon.

On Wednesday, actress ­Angelina Jolie met with Syrian refugees in Lebanon a day after visiting a refugee camp in Jordan.

Jolie, who came along with UN High Commissioner for Refugees Antonio Guterres, held talks with Prime Minister Najib Mikati of Lebanon.

There are more than 65,000 Syrian refugees in Lebanon.