The conflict in Syria has now reached "new and appalling heights of brutality and violence", according to the UN secretary-general, while the world body's envoy to Syria has said the country is in danger of becoming "a failed state".
The statements from Secretary-General Ban Ki-moon and Joint UN-Arab League envoy Lakhdar Brahimi came in New York on Friday, while violence continued near the Damascus international airport and the country's Internet remained blacked out.
Ban and Brahimi were addressing the 193-member UN General Assembly on the revolt against Syrian President Bashir al-Assad, which began as peaceful rallies calling for democracy but grew to an armed struggle after the military cracked down on protesters.
The fighting has killed about 40,000 people, making it the bloodiest of Arab uprisings that have ousted entrenched leaders in Tunisia, Egypt, Libya and Yemen since early last year.
"The government has intensified its campaigns to root out opposition strongholds and has increased shelling and air strikes," Ban said. "Opposition elements also have stepped up their attacks. I am horrified and saddened and condemn the seemingly daily massacres of civilians."
Syrian air force jets bombarded rebel targets on Friday close to the Damascus airport road and a regional airline said the violence had halted international flights. The Internet and most telephone lines were also down for a second day in the worst communications outage of the conflict.
Ban said with the onset of winter, up to four million people in Syria would be in need and that he expected to number of refugees - currently about 480,000 - to hit 700,000 by early next year. He appealed for more humanitarian aid and said he would soon visit refugee camps in Jordan and Turkey to assess the situation.
Brahimi said rebel forces had made gains on the ground in the past few weeks, but the government remained confident that it has the upper hand.
"In Syria itself, there is no trust between the parties. They do not even define the problem in the same terms," he said.
Brahimi told the General Assembly that Syria was in danger of becoming a failed state and stepped up his pressure on the Security Council, which is deadlocked over taking stronger action on Assad, to adopt a resolution backing his peace bid.
Fighting near airport
Meanwhile, violence was reported along the 27km-long road to the Damascus international airport, with the government saying that it had now secured the route and that international flights were free to resume on Friday. Rebels disputed that claim.
"After strong clashes, rebels were able to take control of a part of the airport road between the second and fourth bridge," activists said.
State television had on Thursday night quoted the information ministry as saying that the airport road, closed for much of the day due to the fighting, had been "secured" after military intervention.
Both EgyptAir and Emirates had on Thursday announced suspension of flights due to the fighting.
A Syrian security source said: "[On Thursday], we sent a telegram to the foreign companies still working in Syria and SyrianAir [the national carrier] to inform them of the status of the road to the airport so they would cancel their flights to Damascus."
"Today after the road was secured, we sent a new telegram to inform them that security was restored," she told AFP.
A military source in Damascus said that the army had taken control of the western side of the road leading to the airport and a small portion on the east by dawn, allowing travellers to move through.
"But the most difficult part is yet to come. The army wants to take control of the eastern side, where there are thousands of terrorists and this will take several days," he said, using the term regime officials use to describe rebel fighters.
Meanwhile, at least 17 Lebanese fighters have gone missing after an ambush by government forces inside Syrian territory. The fighters were part of a group of 20, mostly from the Lebanese city of Tripoli, who were travelling to Syria with the intention of joining rebel forces in Homs.
Three of the fighters escaped the ambush and were able to return to Lebanon, Al Jazeera's Mysa Khalaf reported from Beirut.
As violence continued, the blackout of phone and internet networks remained in place across most of Syria for a second straight day.
The length of the internet blackout, which continued on Friday, is unprecedented in Syria's 20-month-old uprising against President Bashar al-Assad.
Renesys, a US-based company that monitors online connectivity, confirmed that "all 84 of Syria's IP address blocks had become unreachable [on Thursday], effectively removing the country from the Internet".
Residents in some areas reported on Friday that they could access the internet but "with great difficulty".
In-depth coverage of escalating violence across Syria
"It is also very difficult to reach people by phone," Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights, reported.
"But we have received reports that it is possible to communicate between certain regions via fixed telephone lines."
AFP news agency correspondents noted that internet and telephone communications, including mobile phones, were cut in the capital.
The official news agency SANA had still not resumed transmission on Friday after its feed was cut on Thursday at midday. The agency's website was also inaccessible.
Google and Twitter announced on Friday that they had reactivated a voice-tweet programme to allow Syrians affected by the shutdown of the Internet to get messages out. The service allows people with a telephone connection to compose and send a tweet by speaking on their phones.