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BEIRUT -- Six days of fighting near Damascus has killed at least 100 people and possibly many more, activists said Monday, in what both sides say may be a dramatic spike in the Syria's civil war death toll.

The reports came as President Bashar Assad's forces pressed an offensive against rebels closing in on parts of the Syrian capital, and government troops moved to encircle the contested town of Qusair near the Lebanese border.

The exact number dead in the Jdaidet Artouz and Jdaidet al-Fadel districts could not be confirmed. The two adjacent neighbourhoods are about 15 kilometres (10 miles) southwest of Damascus.

The Britain-based Syrian Observatory for Human Rights said the death toll, mostly from shelling, could be as high as 250. Rami Abdul-Rahman, who heads the Observatory, said the group has documented 101 names of those killed, including three children, 10 women and 88 men, but he thought the toll would be much higher. The dead included 24 rebels, he said.

The Local Coordination Committees, another activist group, put the death toll at 483. It said most of the victims were killed in Jdaidet Artouz.

Both activist groups, the Observatory and the LCC, rely on a network of activists on the ground in different parts of Syria.

State-run news agency SANA said Syrian troops "inflicted heavy losses" on the rebels in the suburbs.

A government official in Damascus told The Associated Press that rebels were behind the "massacre" in Jdaidet al-Fadel, saying they sought to blame government forces who entered the area after the killings.

"The army discovered the massacre after entering the area," the official said on condition of anonymity in line with regulations. The corpses were already decomposed, he said.

Jdaidet al-Fadel is inhabited mostly by Syrians who fled the Golan Heights after the area was captured by Israel in 1967. Jdaidet Artouz has a large Christian and Druse population -- two minority communities that have generally stood by Assad or on the sidelines.

The killings appeared reminiscent of violence in the Damascus suburb of Daraya in August. At the time activists said days of shelling and a killing spree by government troops left 300 to 600 dead.

Mohammed Saeed, an activist based near Damascus, said rebels withdrew as soon as the government offensive began last week. After that, he said via Skype, troops and pro-government gunmen stormed the area and over several days killed about 250 people.

"The situation is very tense," Saeed said, noting that the area has no electricity, water, or mobile phone service. "There is widespread destruction in Jdaidet al-Fadel, including its only bakery."

Reports of death tolls in Syria's civil war often conflict, especially in areas that are difficult to access because of the fighting. The government also bars many foreign journalists from covering the conflict.

The main opposition group, the Cairo-based Syrian National Coalition, described the killings as "the latest heinous crime committed by the Assad regime." It complained in a statement that "the deafening silence of the international community over these crimes against humanity is shameful."

British Foreign Secretary William Hague said the reports of the massacre underline the urgent need to bring Syria's war to an end.

"I am appalled by the reports of the killing by Syrian Government forces of dozens of people, including women and children, in the town of Jdaidet Al-Fadel, a suburb of Damascus," Hague said in a statement. "This is yet another reminder of the callous brutality of the Assad regime and the terrible climate of impunity inside Syria."

Russian Foreign Minister Sergey Lavrov said Monday he shared with U.S. Secretary of State John Kerry "a disposition to seek a political outcome as soon as possible and look for ways to transfer this situation into a channel of negotiations between the government and the opposition" in Syria. The two spoke Saturday.

Lavrov said he and Kerry would discuss what the U.S. and Russia could do to "induce those who are currently resisting the peace process to change their position" at the NATO-Russia summit in Brussels on Tuesday.

Lifting the European Union embargo on supplying weapons to the Syrian opposition would violate legal obligations not to arm non-state actors, said Lavrov, whose country is among Assad's strongest supporters.

In Damascus, Assad discussed the crisis with Alaeddin Boroujerdi, head of Iran's parliamentary committee on national interest and foreign policy. State-run news agency SANA quoted Assad as saying during the meeting that the Middle East is being "subjected to plans that target its stability and unity of its territories."

Boroujerdi said later that the best solution to Syria's crisis is for Assad to stay in power until his term ends in the summer of 2014, "when there would be free elections and the Syrian people would say their word."

The Syrian opposition refuses to talk to government officials as long as Assad remains in power. Syrian officials have said Assad, who has been in power since 2000, will run for the post again in next year's elections.

Also Monday, two bombings targeted an army checkpoint and a military post in a third Damascus suburb, Mleiha, killing eight soldiers there, according to the Observatory.

SANA said a suicide attacker detonated a car bomb in Mleiha, injuring several people, including students heading to schools and universities.

The army also pressed on with its offensive near the Lebanese border, where it has been pushing for two weeks to regain control along with the help of a Hezbollah-backed militia known as the Popular Committees. The region is strategic because it links Damascus with the Mediterranean coastal enclave that is the heartland of Assad's Alawite sect, an offshoot of Shiite Islam.

The fighting around Qusair also points to the sectarian nature of the Syrian conflict, which pits a government dominated by the president's Alawite minority against a primarily Sunni Muslim rebellion, and underscores widely held fears that the civil war could drag in neighbouring states.

In Lebanon, there are deep divisions over the Syrian conflict, with Lebanese Sunnis mostly backing the opposition while Shiites support Assad. Lebanese fighters have also travelled to Syria to join either Sunni or Shiite groups, and several have been killed in clashes.