Syrian government troops trying to relieve a besieged military base in the country's northwest ambushed a rebel checkpoint Saturday, killing at least 12 opposition fighters, activists said.

The Britain-based Syrian Observatory for Human Rights said the government forces surprised the rebel fighters on the outskirts of the village of Baboulein in Idlib province. The Observatory, which relies on a network of local activists on the ground, said many opposition fighters were also wounded in the attack.

Rebels have wrested much of the countryside of Idlib and other provinces in northern Syria from regime forces, although government troops still control many military bases in the region from which they launch attacks on opposition-held areas.

Observatory director Rami Abdul-Rahman said the assault Saturday was part of government efforts to resupply the besieged military base at Wadi Deif outside of the town of Maaret al-Numan, which is just north of Baboulein on the Damascus-Aleppo highway.

Rebels have been trying for months to capture the large base at Wadi Deif, from which regime troops regularly pound the now largely abandoned town of Maaret al-Numan with artillery fire. The regime must push convoys through rebel-held territory to prevent the base from running short of troops and supplies.

On Thursday, activists said rebels shot down a helicopter carrying food and supplies to the base, killing the pilot and three other soldiers.

In the northern city of Aleppo, a government air raid on the disputed Sheikh Maqsoud neighborhood killed at least three people and wound 16 others, the Observatory said. It added that doctors treating the wounded said many showed signs of inhaling toxic gas, such as severe vomiting, runny nose and irritation of the eyes.

Both sides in the Syrian civil war have accused the other of using chemical weapons.

Syria has asked the U.N. to investigate an alleged chemical weapons attack by rebels in March on the village of Khan al-Assal outside of Aleppo. The rebels blame regime forces.

Britain and France want the U.N. to investigate allegations of chemical weapons use in Khan al-Assal and another village, Ataybah, on March 19, as well as the central city of Homs on Dec. 23.

Syria has rejected U.N. Secretary-General Ban Ki-moon's push to expand the U.N. probe to include those other villages.

The Syrian conflict began in March 2011 with largely peaceful protests but has since devolved into a civil war that the United Nations says has killed at least 70,000 people. Hundreds of thousands of Syrians have sought refuge abroad, and millions inside the country have fled their homes to try to find safety elsewhere inside Syria.

International efforts to find a peaceful resolution to the conflict have faltered.

U.N.-Arab League envoy Lakhdar Brahimi is scheduled to address the Security Council on Thursday. Brahimi has not been able to make progress in his mission to push forward a peace plan for Syria first presented in June at an international conference in Geneva.

On Saturday, Syrian state-run daily Al-Thawra accused Brahimi of being a "false witness,"and said he has taken sides in the conflict and his briefing "will not alleviate the suffering of Syrians."

Brahimi angered the Syrian government by saying in December that the four-decade rule of the Assad family had gone on "too long."