BEIRUT -- Syrian rebels strengthened their hold Thursday on an oil-rich province bordering Iraq, activists said, capturing a key military base that was considered one of the last bastions for President Bashar Assad's loyalists in the strategic region.

The reported fall of the Mayadeen base, along with its stockpiles of artillery, caps a series of advances in Deir el-Zour including last week's seizure of a military airport.

The province borders on western Iraq. Syria's rebels enjoy strong support with the Sunni tribes of Iraq's west, and many Iraqis with combat experience from their own war are believed to have crossed to fight in their neighbor.

Rebel fighters also say that weapons seized when bases fall have been essential to their transformation from ragtag brigades into forces capable of challenging Assad's professional army.

Activist groups and a local fighter told The Associated Press the Mayadeen base was taken in the morning hours, after a three-week siege. The fighter spoke on condition of anonymity because of security concerns.

Violence also was reported in opposition strongholds around the capital Damascus and in the northern city of Aleppo, where government aircraft damaged one of the rebels' key field hospitals.

Rebels who have battled government forces for months to control Aleppo, Syria's economic hub, scored a major victory several days ago when they overran the nearby base of the regime's 46th Regiment. The unit was a pillar of the government's Aleppo garrison and its fall cuts a major supply line.

However, the regime has used its air power to dent rebel gains. Government aircraft late Wednesday flattened a building next to Dar al-Shifa hospital, killing 15 people and badly damaging one of the last remaining sources of medical help for civilians in the city, activists said.

Once a private clinic run by a businessman said to be close to Assad, Dar al-Shifa became a field hospital run by volunteer doctors, nurses and aides united by their opposition to the regime. They gave medical care to both civilians and rebels.

The facility has taken at least six direct shell hits in recent months, mostly affecting the upper floors. The seven-story hospital is only 400 to 500 yards from the front line in a neighborhood that is heavily shelled every day.

The warplanes turned the building adjacent to the hospital into a pile of rubble and sprayed shrapnel and debris into Dar al-Shifa itself, activists said.

Rami Abdul-Rahman, chief of the British-based Syrian Observatory for Human Rights, said at least 11 fighters were killed in the raid, in addition to a doctor, a young girl and two children who were on the street.

Another activist group, the Local Coordination Committees, confirmed the bombing and identified the doctor as Mohammad Qassem Agha. The group said 40 people died in airstrikes in Aleppo on Wednesday, but did not say how many died in the hospital strike.

Videos posted online by activists showed the flattened building. Residents and rebels along with a doctor in green scrubs are seen picking through the rubble and overturned gurneys outside the hospital entrance.

In one video, a man calls out to survivors under the rubble, while one of the survivors is heard crying for help from beneath a huge slab of concrete.

In Damascus, two mortar shells struck the upscale neighborhood of Mazzeh during the morning rush hour Thursday. An AP reporter said one of the shells set fire to a six-floor apartment in a residential building, seriously injuring one woman. The second mortar struck and damaged the first floor in a building across the street.

Downtown Damascus -- the seat of Assad's power -- has seen scores of car bombs and mortar attacks in recent months. Mazzeh, home to a number of foreign embassies as well as homes of wealth Syrians, including one exclusive compound housing members of the regime, has been targeted several times in the past few days.

"This is a residential area and there are no military bases here. So why are they targeting civilians?" said Nizar Hamdi, a 38-year-old owner of a computer center.

Syrian TV showed a girl in school uniform who said the mortar fell as she was preparing herself to go out.

"It was terrifying, I couldn't go to school. People were screaming," she said.

The state-run SANA news agency also reported that a car bomb exploded in the Massaken Barzeh district of the capital, wounding another person.

The reports blamed "terrorists" for the attacks, a term the government uses for opposition fighters.

Meanwhile, the military pounded opposition strongholds in the outskirts, activists said. In videos that were posted online by activists Thursday, mortar rounds and artillery shells can be heard landing in the suburb of Daraya. Plumes of black smoke are seen rising from behind rows of houses in a residential area and a fire engulfs a one of the buildings that was hit.

With a population of about 200,000, Daraya is part of Rural Damascus, a province that includes the capital's suburbs and farmland. It has been a stronghold of support for the rebels fighting the government since the start of the uprising, posing a particularly grave threat to Assad's seat of power.

In August, troops backed by tanks stormed the town after several days of siege, with hundreds reportedly killed.

To the north, near the border with Turkey, fighting broke out in the city of Ras al-Ayn on the Syrian side of the border between Kurdish and Arab rebel factions, according to an official at the mayor's office in the nearby Turkish town of Ceylanpinar. He said two wounded rebels were brought over to Turkey for treatment, but he did not say to which faction they belonged. The official spoke on condition of anonymity in line with government rules.

Kurdish and Arab groups cooperated to oust Syrian regime forces from the ethnically mixed area earlier this month, but they have since frequently clashed over control of the city.

The Syrian Observatory for Human Rights, which relies on reports from the ground, confirmed the infighting.

Syria's conflict erupted in March 2011 with an uprising against Assad's regime, inspired by other Arab Spring revolts. The crisis has since morphed into a civil war, with scores of rebel groups across the country fighting government troops. More than 40,000 people have been killed in the 20 months of unrest, according to activists.