Syrian troops and rebels clashed inside the tightly controlled capital Monday for the second day in what activists called some of the worst fighting since the country's crisis began 16 months ago. The clashes briefly closed the highway linking the capital with Damascus International Airport to the city's south - an unprecedented development, said Mustafa Osso, an activist based in Syria. "It seems there is a new strategy to bring the fighting into the center of the capital," Osso told The Associated Press, referring to the rebels who fight under the banner of the Free Syrian Army. The capital used to be safe. This will trouble the regime. The Britain-based Syrian Observatory for Human Rights and Osso said the fighting was concentrated in the districts of Kfar Souseh, Midan and Tadamon. An amateur video posted online showed Kfar Souseh as the sound of intense gunfire could be head in the distance. The presidential palace, on a mountain overlooking the capital, could be seen in the background. The authenticity of the video could not be independently confirmed. There have been sporadic clashes in Damascus in recent months, although President Bashar Assad's forces remain firmly in control of the capital. Many of the Damascus suburbs, however, have risen up against the regime, prompting a ferocious response from the military in an attempt to clear out rebel fighters from the towns that ring the capital. On Monday, activists reported government attacks in the Damascus suburb of Qatana. Rami Abdul-Rahman, who heads the Observatory, said Mondays' fighting was taking place about 2 kilometers (1 mile) from the airport highway. A Damascus resident who spoke on condition of anonymity for fear of reprisals said gunfire and sporadic explosions could be heard throughout the morning. The resident said that unlike previous clashes that occurred at night, the recent fighting took place during the day - a sign that the rebels are becoming more brazen. The clashes come as Russian Foreign Minister Sergey Lavrov on Monday accused the West of using blackmail to secure a new U.N. Security Council resolution that would authorize the use of force in Syria. A Chapter 7 resolution authorizes actions to enforce sanctions that can ultimately include the use of military force, which U.S. administration and European officials - for now - are playing down as a possibility. U.N. special envoy Kofi Annan was expected in Moscow on Monday to discuss the Syria crisis. On Sunday, Syria's 16-month bloodbath crossed an important symbolic threshold as the International Red Cross formally declared the conflict a civil war, a status with implications for potential war crimes prosecutions. Also Sunday, United Nations observers gathered new details on what happened in a village where dozens were reported killed in a regime assault. After a second visit to Tremseh on Sunday, the team said Syrian troops went door-to-door in the small farming community, checking residents' IDs and then killing some and hauling others away. According to the U.N., the attack appeared to target army defectors and activists. "Pools of blood and brain matter were observed in a number of homes," a U.N. statement said. Syria denied U.N. claims that government forces had used heavy weapons such as tanks, artillery and helicopters during the attack Thursday. Anti-regime activists estimate that more than 17,000 people have been killed in Syria since the uprising began.