BEIRUT -- A government warplane bombed an apartment building in eastern Syria Friday, killing at least 21 people as the regime fought to claw back ground lost to rebel fighters in the area who have made significant advances in Mayadin, activists said.

In Damascus, shells from mountains overlooking the Syrian capital crashed into the rebellious suburb of Daraya as part of a days-long regime offensive to regain control of the area.

Activists said at least 15 people were killed in the shelling and clashes.

The air raid on Mayadin, a city in Deir el-Zour province near the Iraqi border, occurred after rebels gained control of a key checkpoint on a bridge over the Euphrates River there, local activist Abu Omar al-Deery said.

He claimed that rebels have largely gained control of Mayadin for the first time in the 17-month-old uprising, adding that the only part still in regime hands is an artillery position on a hill overlooking the city.

Rebels seeking to oust President Bashar Assad have been fighting to expand their foothold along the eastern frontier. The opposition already controls a wide swath of territory along the border with Turkey in the north, as well as pockets along the frontier with Jordan to the south and Lebanon to the west.

The Local Coordination Committees, a key activist group, and the British-based Syrian Observatory for Human Rights said 21 people were killed in the airstrike. The British group said the number included 12 women and a child. Al-Deery put the death toll at 23. The figures and details could not be independently confirmed due to tight controls over the media in Syria.

"There was a real massacre in Mayadin. I believe it is to avenge the takeover of the city by the Free Syrian Army," al-Deery said.

Fighting also continued around the southern Damascus suburb of Daraya, the scene of intense clashes this week. The Observatory, which relies on a network of activists on the ground, said 15 were killed there Friday, many from wounds suffered in battles the day before. Human rights groups say more than 20,000 people have been killed in Syria since the uprising against Assad erupted in March 2011.

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