BEIRUT, Lebanon -- Airstrikes by the Syrian government damaged a hospital in the northern city of Aleppo early on Thursday and flattened a building next to it, killing at least 15 people and leaving as many as 40 missing in an attack that closed one of the city's few functioning medical facilities, antigovernment activists said.
Video purporting to depict the aftermath showed the facade shorn off the first three stories of the hospital, with its name, Dar el-Shifa, in red letters on its tower. Beside it, another building was reduced to a two-story pile of rubble. People milled in the street, shouting "God is great."
Among the 15 people confirmed dead were two hospital workers and two children, said Abu Louai al-Halabi, an activist in Aleppo, adding that up to 40 people were still believed to be trapped under the rubble. One man was pulled out alive several hours after the explosion, according to another video posted on the Internet by opponents of President Bashar al-Assad.
Rebels seized a military base in southeastern Syria, giving them control of an area of oil-producing territory, and as tensions increased between antigovernment fighters and Kurdish groups in northeastern Syria. In both areas, near the border with Iraq, activists said that fighters from Al Nusra Front for the People of the Levant, a jihadi group, were taking a prominent role among the opposition fighters.
The developments came a day after a well-known antigovernment activist was arrested during a bold protest that showed that the nonviolent opposition movement is still struggling to survive even as civil war deepens.
In the old market in central Damascus, the activist, Rima Dali raised a banner calling for "the end of all military operations" -- an act of extraordinary defiance during a time of tight security and surveillance in the capital.
Ms. Dali and three other women stood in wedding dresses in the middle of the arched Souk al-Hamediya, usually bustling with spice sellers but apparently nearly empty. Photographs were posted on Ms. Dali's and other activists' sites.
"Syria is for all of us," the banners read. "You are tired and we are tired. We want to live. Another solution ..." A video e-mailed by another activist appeared to show the women being led away by security forces.
Ms. Dali had been arrested in March for standing silently in front of Parliament with a sign reading, "Stop the killing" and calling for "Syria for all Syrians."
Such acts of civil disobedience have been eclipsed as the Syrian protest movement grew into a civil war that has killed more than 30,000 people, but beneath the surface tensions still ripple between rebel leaders and government opponents who favor a less violent approach.
Yet fighting raged in eastern Syria, threatening to engulf displaced people who fled there from other cities, and in the northeast, activists reported tensions between jihadist fighting groups and both pro-regime and antigovernment Kurds.
In Ras al-Ain, near the Turkish border in northeastern Syria, activists said clashes broke out between antigovernment battalions and Kurds from the P.Y.D., or Kurdish Democratic Union Party. That group has ties to a militant Kurdish group long backed by the Syrian government.
But the conflict was not a clear-cut one between government opponents and supporters, an Arab activist who fled to Turkey from Ras al-Ain said in a telephone interview.
The activist, who gave only his first name, Miral, for security reasons, said that the antigovernment battalions in the area did not represent the mainstream of the Syrian uprising, but were jihadist battalions, including from the Nusra Front. He said they enjoyed strong logistical support from similar groups inside Iraq as well as Arab tribes that straddle the border between the two countries.
He said there was little military reason for a heavy rebel presence in the area and suspected that the fighters simply wanted to establish a base in a remote area of Syria. Kurds and Arabs alike are eager to control the area if the government falls.
Christians and Kurds in the area, he said, had agreed to keep extremists out after some fighters burned shops selling alcohol.
"It's a conspiracy against the Kurds," said another Arab activist who fled to Turkey, Sobhi Dawood. He said he was concerned that all sides were building up arms supplies, threatening to fuel "a civil war in the town."
Because some residents disagree with the goals of the jihadi battalions, he said, it has been hard to recruit locals to fight against the government. He said 62 recent recruits had quit, saying "We are not here to fight against our brothers."
In Eastern Syria, a McClatchy correspondent in the province of Deir al-Zour reported that rebels appeared to control two of the three oil fields there, siphoning light crude to burn for heat and to sell, and robbing the government of key revenues.
In a video posted online, a fighter claimed that rebels had "liberated" an artillery base in Mayadeen, outside the province's main city, capturing a tank and a tank carrier.
"I hope Assad's booty and weapons and their money and their land will be ours, God willing," he said, concluding with a reference to the prophet: "Our master is Muhammad, our commander forever."
Hajj Abu Bakr, a local activist in Deir al-Zour reached through Skype, said that three battalions, including one from the Nusra Front, had taken part in seizing the artillery battalion.
He said that clashes in the area had continued for 22 days. He said government shelling had intensified after the artillery position was captured, and that a family of six had been killed. Many families from other provinces, like Homs, are sheltering in the area and he said he was worried that more of the displaced would be injured.
Hwaida Saad contributed reporting.
This article originally appeared in The New York Times.