Rebel fighters gunned down a helicopter in a battleground town near Damascus on Thursday, a watchdog said, as Syria's opposition declared parts of the capital a "disaster area."
A series of explosions rocked Douma, just northeast of Damascus, shortly before the rebels downed the helicopter, said the Syrian Observatory for Human Rights. State television said the aircraft had "crashed".
The latest escalation of violence in the 18-month uprising came as Dutch Foreign Minister hosted a "Friends of Syria" meeting in The Hague to refine sanctions against the regime of President Bashar al-Assad.
Overnight, UN chief Ban Ki-moon warned that the Syrian government and rebels seemed intent on fighting to the bitter end, while saying the international body may offer a new strategy for peace.
On Thursday, helicopter gunships pounded Al-Hajar Al-Aswad, as the opposition Syrian National Council (SNC) added new claims of devastation to the southern district of Damascus and in adjacent neighbourhoods.
The Syrian Revolution General Commission (SRGC), a grassroots network of anti-regime activists, made the same claim on Wednesday, and pleaded for help from international aid organisations.
"Helicopter gunships are pounding civilian homes in Al-Hajar Al-Aswad in southern Damascus," the SNC, Syria's main opposition coalition, said in a statement.
"Many people have been killed or injured, but the violence of the shelling is making it difficult for activists in the area to document all their names," it added.
On Wednesday, the Britain-based Observatory reported 12 people killed in the two southern districts.
"We call on the heroes of the (rebel) Free Syrian Army to intervene and to target the army of Assad," said the SNC.
"We also call on them to open routes for the civilians to flee the catastrophic conditions they are living in."
The SNC also renewed its call on the international community to intervene on behalf of the Syrian people.
"The international and Arab response to what is happening in the world's oldest capital city (Damascus) has been completely insufficient," it said.
On Wednesday, the SRGC had said that, "since July 15, these neighbourhoods have suffered fierce army assaults, as well as indiscriminate shelling targeting civilian homes and shops."
"People who fled the violence in search of safety have paid the highest price," it added.
The SRGC accused the regime of "carrying out a series of summary executions" in the south of the capital, adding that at least 200 people have been killed in the afflicted districts since violence broke out in the area mid-summer.
Speaking in New York on Wednesday, the UN secretary general said the conflict would top the agenda at next week's UN General Assembly summit of world leaders even though there is no formal meeting on Syria.
"Unfortunately both sides, government and opposition forces, seem to be determined to see the end by military means," Ban told reporters.
"I think military means will not bring an answer," he said, calling for "political dialogue reflecting the genuine aspirations and will of the Syrian people."
Ban said UN-Arab League envoy Lakhdar Brahimi, who just wrapped up his first visit to Syria and its neighbours since taking up his post earlier this month, may put a plan to Assad's government after next week's UN talks.
Iranian Foreign Minister Ali Akbar Salehi called for a Syrian solution to the conflict as he visited Damascus for talks with Assad.
Salehi, whose country is closely allied to the Assad regime, said Syria was "facing a problem, and we hope that this problem can be solved as soon as possible."
But he added that "Syria has very strong, solid ties with Iran, especially at the political level."
Earlier this week, Salehi appealed for a simultaneous halt in clashes and violence by both sides in the conflict, while insisting on a peaceful solution without foreign intervention and a halt to aiding the Syrian opposition.
In other developments, the new UN envoy on children in conflict, Leila Zerrougui, said the world body was investigating Syrian rebel groups as well as government forces for attacks that have killed children.
"We have received information concerning indiscriminate bomb attacks which have killed children in Damascus and other areas, and continue to document incidents committed by armed actors, such as the Syrian Free Army, who may have children associated with their forces," Zerrougui told the UN Security Council.
More than 27,000 people have been killed in violence across Syria since March last year, according to the Observatory. The United Nations puts the figure at more than 20,000.