BEIRUT, Lebanon -- As fighting raged in the suburbs of the Syrian capital, Damascus, and gunfire could be heard from the city center, rebels seeking the overthrow of President Bashar al-Assad were reported on Friday to have declared the capital's main airport a "fair target," warning travelers that they used it at their peril.
Against the backdrop of battlefield uncertainty, diplomacy also seemed to have made little perceptible progress. Secretary of State Hillary Rodham Clinton repeated calls for the ouster of President Assad, but said there had been no "great breakthrough" in talks she held Thursday in Dublin with her Russian counterpart, Sergey V. Lavrov, and Lakhdar Brahimi, the special Syria envoy of the United Nations and the Arab League.
"It was an important meeting, but just the beginning," she said, speaking in Belfast, Northern Ireland, before flying back to Washington.
"I don't think anyone believes that there was some great breakthrough. Nobody should have any illusions about how hard this remains. But all of us with any influence on the process, with any influence on the regime or the opposition, needs to be engaged with Brahimi for a concerted, sincere push to see what is possible."
"The advancing developments on the ground," she said, "are increasingly dangerous."
"The United States stands with the Syrian people in insisting that any transition process result in a unified, democratic Syria in which all citizens are represented," Mrs. Clinton continued. "And a future of this kind cannot possibly include Assad. So we go into these discussions with a clear sense of what we want to see accomplished but a realistic understanding of how difficult it still is."
Russia has been Mr. Assad's most durable backer throughout the crisis and has resisted efforts to push him out of power. After the Dublin talks, Mr. Lavrov was quoted as saying that he would not make "optimistic predictions" and that Mr. Brahimi, the special envoy, knows that the chance of success is "far from 100 percent."
The bleak assessment came as government and rebel forces were locked in sustained battle, particularly to the south of the capital where, in recent days, the airport has been caught up in fighting for the capital's suburbs and has been closed to civilian flights for days at a time.
Apart from its importance as a logistical center, the airport, 12 miles south of the capital, holds symbolic value. Its loss would boost the rebels' ability to depict Mr. Assad as isolated and beleaguered.
Nabil al-Amir, a spokesman for an insurgent military group attacking the airport south of Damascus, said rebels "who have been putting the airport under siege decided yesterday that the airport is a fair target," Reuters reported.
"The airport is now full of armored vehicles and soldiers," Mr. Amir said, seeming to suggest that it was firmly in government control. "Civilians who approach it now do so at their own risk."
News reports also suggested that government forces were seeking to bring in reinforcements for a counterattack designed to reverse rebel gains on the fringes of the city.
The rebel threat seemed to deepen the uncertainties of the military campaign for Damascus where visiting reporters say that the sound of government artillery fire pounding outlying suburbs is clearly audible from the city center -- once a haven of tranquillity even as the uprising against Mr. Assad evolved from peaceful protest in March 2011 to civil war.
Activists said government forces backed by tanks were heading toward two southwestern suburbs, covering their effort to advance with rocket and mortar fire.
Overnight, sounds of gunfire were heard in central Damascus near a major road, Baghdad Street.
On the southern edge of the city, in Tadamon, where antigovernment sentiment is strong and clashes have taken place all week, rebel fighters took control of a checkpoint, the Local Coordinating Committees, an antigovernment activist network, reported.
In the central city of Homs, a car bomb exploded just before noon near a mosque in the wealthy residential area of Inshaat, neighboring the restive Baba Amr neighborhood, and many people were reported injured, residents and activists said.
There was no immediate claims of responsibility, but a demonstration denouncing the government broke out shortly afterward.
"I woke to the explosion," a 35-year-old resident said, speaking in return for anonymity. "When I went down I saw a car on fire. The guys started a demonstration and we started chanting. Ten minutes later the security started shooting at us, so we ran away. Now I'm sure we should be armed next time."
The Local Coordinating Committees also reported an explosion in Hama that it blamed on pro-government militias.
On Thursday, a car bomb exploded in the southern Zahraa district of Damascus, killing one person and damaging the headquarters of the Syrian Arab Red Crescent relief organization, state news media reported.
The official SANA news agency blamed terrorists, its usual designation for opponents of Mr. Assad.
The Syrian Arab Red Crescent plays a crucial role in distributing food to displaced Syrians and works with the United Nations World Food Program, which announced Tuesday that its efforts were being disrupted by attacks on delivery vehicles.
Another car bomb on Thursday shook the Damascus neighborhood of Mezze 86 for the second time in less than three weeks, the Syrian Observatory for Human Rights reported. The neighborhood, on the southwestern edge of the city, is home to many Alawites from the Syrian coast, including military family members. The observatory said there was heavy damage in the area but no casualties.
Fighting continued throughout the Damascus suburbs, where government forces attacked rebel-held neighborhoods with artillery fire, and battles raged along the road to the airport as security forces continued a counteroffensive against opposition forces that in recent weeks have tried to encircle Damascus and cut off the airport road.
Residents on Thursday said central Damascus was locked down and that its people were afraid, with traffic jams and checkpoints making it difficult to move about the city, and the sounds of explosions throughout the day and night.
North of Damascus, rebels continued to fight government forces around two key bases: Wadi al-Deif, at a critical crossroads on the road between Damascus and Aleppo, and the Managh military airport in the northern province of Idlib, where a standoff has continued for months between government forces and rebel fighters who have cut off road access to the base.
Here in Lebanon, Reuters reported, two men were killed by snipers in the northern port of Tripoli on Thursday during sectarian clashes between gunmen loyal to opposing sides in Syria's civil war. They were the latest fatalities in three days of clashes that have killed 8 people and wounded 58.
Anne Barnard reported from Beirut, Alan Cowell from London and Michael R. Gordon from Belfast, Northern Ireland. Hala Droubi contributed reporting from Dubai, United Arab Emirates.world
This article originally appeared in The New York Times.