BEIRUT (AP) — A fighter jet bombed an apartment building Friday in eastern Syria, killing at least 21 people after rebels in the area made significant advances in the city and took control of a major checkpoint, activists said.

In Damascus, shells from mountains overlooking the Syrian capital crashed into the rebellious suburb of Daraya as part of a days-long government 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 in Syria, adding that the only part still in the hands of the regime is an artillery position on a hill overlooking the city.

Rebels seeking to oust Syrian 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 British-based Syrian Observatory for Human Rights said 21 people, including 12 women and a child, were killed in the airstrike. 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 told The Associated Press through Skype.

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 people died there Friday, many from injuries suffered in battles the day before. The Local Coordination Committees, a key activist group, said 21 people were killed in Daraya.

Damascus residents reported hearing loud explosions as shells fired from the Qasioun mountains overlooking the capital slammed into Daraya and the nearby suburb of Moadimiyeh.

Human rights groups say more than 20,000 people have died since the Syrian uprising against Assad erupted in March 2011 and evolved into a civil war. The bloodshed already has spilled over into neighboring countries.

In neighboring Lebanon, fresh clashes broke out Friday in the northern city of Tripoli between supporters and opponents of the Syrian regime, killing two people and wounding 17 others, Lebanese security officials said.

Gunmen from the Alawite neighborhood of Jabal Mohsen fought armed elements from the neighboring district of Bab Tabbaneh, which is populated by Sunnis. The Alawite sect is an offshoot of Shiite Islam. Assad comes from Syria’s Alawite minority, while rebels fighting his regime are mostly Sunnis.

Among those wounded by sniper fire was a technician working with journalists in the area.

Lebanon, a country that suffered its own 15-year civil war from 1975 to 1990, has an explosive sectarian mix of Sunni and Shiite Muslims, Christians and Palestinian refugees, as well as deep divisions between pro- and anti-Syrian factions.

Syria was in virtual control of its smaller neighbor for many years, posting tens of thousands of troops in Lebanon, before withdrawing under pressure in 2005. Even without soldiers on the ground, Syria remains influential, and its civil war has stirred longstanding tensions that have lain under Lebanon’s surface.

Clashes over the past few days in Tripoli represent some of the most serious fighting in Lebanon in several months. The mostly Sunni city weathered gunbattles in May, when fighting over Syria killed eight people.

Tensions surged Monday. Several attempts to mediate a cease-fire have failed. In all, 17 people have been killed in the city this week, and more than 100 have been wounded.

Meanwhile, the wife of a journalist for a U.S.-funded television network who was reported missing in Syria said Friday he is believed to be in the custody of pro-government forces there. Alhurra TV correspondent Bashar Fahmi, a Jordanian citizen of Palestinian origin, and his Turkish cameraman, Cuneyt Unal, are said to have been captured in the city of Aleppo after entering Syria on Monday.

Fahmi’s wife, Arzu Kadumi , said she had been informed of witness reports that her husband was seen alive.

Turkish journalist Murat Can, who has investigated the case, says his contacts informed him that Fahmi was injured in the shoulder, and that he and Unal had been captured by pro-government militiamen known as “shabiha.”

A third journalist, Austin Tice has reported for The Washington Post, McClatchy Newspapers and other media outlets from the Middle Eastern country, is also reported missing in Syria.

His father, Marc Tice, said his son hasn’t been in contact with his editors or his family in Texas in more than a week.

The Washington Post reported that the 31-year-old Tice spent time with rebel fighters in the north after entering Syria from Turkey in May. He then traveled to Damascus, where he was one of the few Western journalists reporting from the capital.

Copyright 2012 The Associated Press.