“A loud explosion was heard at the Mazzeh military airport in Damascus,” the Britain-based monitoring group said. It provided no further details.

Activists however said that the army blocked the road leading to the military airbase while snipers took positions on the rooftops of buildings in the area.

Sporadic clashes between government troops and army deserters have rocked Damascus in recent weeks, ahead of the ceasefire that went into effect on April 12.

Mazzeh, an upscale district which houses many government offices and embassies, is a Damascus neighborhood that has been hit by violence.

The latest report comes as a team of United Nations observers monitoring the shaky cease-fire visited the central Syrian province of Homs yesterday.

Monitors say more than 200 people have been killed in Syria since the cease-fire to which the government and rebels committed themselves. The UN monitors visited Homs ahead of an expected Security Council vote on deploying hundreds of observers across Syria to monitor a shaky cease-fire.

"A team of international observers visited the province of Homs and met the governor," the state-run news agency SANA reported.

The small advance team of monitors, who had previously been prevented from visiting Homs for "security reasons," were able to tour different districts of the city of the same name, including battered Baba Amro.

Regime forces shelled Baba Amro for a month, leaving hundreds dead according to monitors, before retaking it from rebels on March 1. Two Western journalists were among those killed.

The visit came as the opposition Syrian National Council claimed that Homs neighborhoods were being pounded, although an activist in the city said the situation was calm.

But in the town of Qusayr, near the Lebanese border in Homs province, a sniper shot dead a woman yesterday, said the Syrian Observatory for Human Rights.

Also in the runup to the UN vote, state-media reported that authorities released 30 people detained for their alleged role in Syria's anti-regime uprising, but who have "no blood on their hands." The move takes to nearly 4,000 the number of people the authorities have released since November, SANA reported. In New York, diplomats said the Security Council had reached a tentative accord on a resolution to send in the 300-strong cease-fire observer force that could be voted later.

Russia's UN envoy was upbeat about the text, although his US counterpart indicated a vote was not certain as Western nations decide if the conditions for the force are strong enough.

The council approved an advanced mission of 30 observers and seven are already in Syria where the UN says more than 9,000 people have been killed in a 13-month crackdown on dissent against President Bashar Assad's rule. Sporadic clashes between government troops and army deserters have rocked Damascus in recent weeks, ahead of a cease-fire that went into effect on April 12. Elsewhere, an armed group yesterday blew up a section of an oil pipeline in the Deir Ezzor region of northeast Syria, SANA said.

On Friday, violence persisted on the ground, with at least 46 people killed as thousands of Syrians protested against Assad's regime, according to monitors and activists.

Meanwhile, Amnesty International expressed concern about the fate of a cardiologist, Dr. Mahmud Al-Rifai, arrested in Damascus on Feb. 16 and "believed to have been tortured" for having treated injured protesters.

The London-based group raised similar fears about another doctor, Mohammed Al-Ammar, whom it said was a peaceful activist detained on March 19 in the southern city of Daraa, cradle of the uprising. On the political front, Russia's UN envoy Vitaly Churkin called for a "unanimous vote" on the text to dispatch monitors to Syria, which his country took a leading role in drawing up.

But his US counterpart Susan Rice indicated a vote is not certain yesterday at the Security Council as the West decides whether the conditions for the force are strong enough.