* South Damascus pounded by tank, helicopter, shell fire -activists * Moscow believes Syria won't use chemical weapons -report * Urban warfare continues unabated in biggest city Aleppo * Fighting rages for control of airfield near Iraq border * Death toll in related Lebanon fighting rises to 10 By Khaled Yacoub Oweis and Oliver Holmes AMMAN/ALEPPO, Syria Aug 22 (Reuters) - Syrian army shells crashed into southern Damascus on Wednesday and helicopters fired rockets and machineguns during an assault to shore up President Bashar al-Assad's grip on the capital 17 months into a popular uprising, opposition activists said. The army has used tanks and helicopter gunships this week in an offensive around Damascus that has coincided with the departure of U.N. military observers after a failed mission to stop the bloodshed and nudge Syria toward a peaceful transition. The United Nations estimates that 18,000 people have been killed in what has become a civil war after a violent state response to peaceful street protests generated an armed rebellion in the pivotal Arab country. Anti-Assad activists said at least 47 people had been killed in Damascus in what they called the heaviest bombardment this month. "The whole of Damascus is shaking with the sound of shelling," said a woman in Kfar Souseh, one of several districts hit during the military offensive to root out rebel fighters. At least 22 people were killed in Kfar Souseh and 25 in the nearby district of Nahr Eisha, activists said. One of the dead was named as Mohammad Saeed al Odeh, a journalist employed at a state-run newspaper who was sympathetic to the anti-Assad revolt. Activists said he had been executed in Nahr Eisha. "There are 22 tanks in Kfar Souseh now and behind each one there are at least 30 soldiers. They are raiding houses and executing men," an opposition activist in Kfar Souseh, who gave his name only as Bassam, told Reuters by Skype. More than 250 people, including 171 civilians, were killed across Syria on Tuesday, mostly around Damascus, Aleppo and the southern city of Deraa, according to the Syrian Observatory for Human Rights, a British-based opposition monitoring group. Activists in the southwestern Damascus suburb of Mouadamiya said Assad's forces had killed 86 people there since Monday, half of them by execution. It was not possible to verify that report. There was no immediate government account of the latest fighting. But state television broadcast footage of weapons it said had been seized from rebels in Mouadamiya, which was one of the first districts to join the uprising. The conflict, which pits a mainly Sunni Muslim opposition against a ruling system dominated by Assad's Alawite minority, threatens to destabilise neighbours including Lebanon, where Sunni-Alawite violence flared for a third day. The death toll from the fighting in the northern Lebanese city of Tripoli rose to at least 10 with more than 100 wounded, medical sources said, in what residents said were some of the fiercest clashes there since Lebanon's 1975-90 civil war. The Syria conflict has revived old tensions in Tripoli between pro-Assad Alawites in the hilltop district of Jebel Mohsen and their Sunni neighbours in Bab al-Tabbaneh below. ALEPPO BATTLES In Syria, Assad's forces have lost swathes of territory in recent months, but have fought back hard in Damascus and in Aleppo, the country's biggest city and commercial hub until it became a theatre for urban warfare. Reuters journalists in Aleppo heard gunfire and shells exploding every minute. Rebel fighters trying to advance in Saif al-Dawla, a front-line Aleppo district, encountered mortar and rocket-propelled grenade barrages. At one point, their escape route was cut off by gunfire as tank shells exploded nearby. Much of the area was destroyed. Water poured from one building into the street below. State television said government forces were pursuing "the remnants of armed terrorist gangs". Syrian government forces also fought rebels on Wednesday for control of a military base and airfield near the eastern town of Albu Kamal on the Iraqi border, according to a local Iraqi official and a Syrian rebel commander. Opposition sources said Syrian state forces had evacuated two security installations at Albu Kamal on the Iraqi border on Tuesday as rebels made gains after a week of heavy fighting. They identified the installations as belonging to the Airforce Intelligence and Political Security agencies in Albu Kamal, 120 km (75 miles) southeast of the city of Deir al-Zor. The rebel commander, known as Abu Khalid, said his forces now controlled Albu Kamal, straddling a supply route from Iraq where many Sunni tribes sympathise with their Syrian kin fighting Assad's forces. As Syria slips deeper into chaos, the United States and Israel have voiced concern that Assad might lose control of his chemical weapons arsenal or even be tempted to use it. Russia, a Syrian ally since Soviet times, believes Syria has no intention of using its chemical weapons and is able to safeguard them, the Russian newspaper Kommersant reported on Wednesday, citing an unidentified Foreign Ministry official. A "confidential dialogue" with the Syrian government on the security of the arsenal has convinced Russia that "the Syrian authorities do not intend to use these weapons and are capable of keeping them under control themselves," Kommersant reported. U.S. President Barack Obama threatened Assad on Monday with "enormous consequences" if he employed chemical weapons or even if he moved them in a menacing way, drawing a warning from Russia against any unilateral action by the West. Russia and China have repeatedly vetoed U.N. Security Council resolutions that Western and some Arab countries had hoped would pile pressure on Assad to end the conflict. Moscow accuses those nations of prolonging the war by backing rebels, whom it often identifies as Islamist militants.