* Foreign ministry says Brahimi is "flagrantly biased"
* U.N.-Arab League envoy's peace efforts make little headway
* Rebels attack air base in northwestern Syria
Syria denounced international envoy Lakhdar Brahimi as "flagrantly biased" on Thursday, casting doubt on how long the U.N.-Arab League mediator can pursue his peace mission.
The Syrian Foreign Ministry was responding to remarks by Brahimi a day earlier in which he ruled out a role for President Bashar al-Assad in a transitional government and effectively called for the Baathist leader to quit.
"In Syria...what people are saying is that a family ruling for 40 years is a little bit too long," Brahimi told the BBC, referring to Assad, who inherited his post from his father Hafez al-Assad, who seized power in 1970 and ruled for 30 years.
"President Assad could take the lead in responding to the aspiration of his people rather than resisting it," the veteran Algerian diplomat said, hinting the Syrian leader should go.
The Foreign Ministry in Damascus said it was very surprised at Brahimi's comments, which showed "he is flagrantly biased for those who are conspiring against Syria and its people".
Brahimi has had no more success than his predecessor Kofi Annan in his quest for a political solution to a 21-month-old conflict in which more than 60,000 people have been killed.
British Foreign Secretary William Hague warned that violence in Syria might worsen and said the international community must "step up" its response if it does.
So far regional rivalries and divisions among big powers have stymied any concerted approach to the upheaval, one of the bloodiest to emerge from a series of revolts in the Arab world.
Russian and U.S. diplomats, who back opposing sides of the war, will meet Brahimi in Geneva on Friday.
"MASK OF IMPARTIALITY"
Syria's al-Watan newspaper daily said Brahimi had removed his "mask of impartiality" to reveal his true face as a "a tool for the implementation of the policy of some Western countries".
On Sunday Assad, making his first public speech in six months, offered no concessions and he said he would never talk to foes he branded terrorists and Western puppets.
As peace efforts floundered, rebels battled for a strategic air base for a second day, pursuing a civil war that had briefly receded for some Damascus residents who set aside their differences to play in a rare snowfall that blanketed the city.
For a few hours, people in the capital dropped their weapons for snowballs and traded hatred for giggles.
"Last night, for the first time in months, I heard laughter instead of shelling. Even the security forces put down their guns and helped us make a snowman," Iman, a resident of the central Shaalan neighbourhood, said by Skype.
There was no respite on other battlefronts, with heavy fighting around the Taftanaz base in northwestern Syria, which insurgents are trying to capture to extend their grip on Idlib province and weaken Assad's control of the skies.
Rebels assaulted the airport's main buildings and armoury using heavy guns, tanks and other weapons and appeared to have overrun half the area of the base, said Rami Abdelrahman, director of the Syrian Observatory for Human Rights, a pro-opposition group that monitors the conflict from abroad.
"Now, it's serious," he said.
The air base has been used to launch helicopter attacks in the region, and its loss would be a blow to the government's ability to defend its positions there, Abdelrahman said.
Insurgents have tried to take the base for months, but have been bolstered by the recent arrival of Islamist fighters including the al Qaeda-linked al-Nusra Front, he added.
There was no immediate government account of the fighting, which could not be confirmed independently.
Opposition forces have seized swathes of territory in northern Syria in recent months, but remain vulnerable to attack by the military's planes and helicopters - hence their strategy of trying to capture air bases such as the one at Taftanaz.
There was no word on whether the firing of a short-range ballistic missile inside Syria on Wednesday, reported by a NATO official, was linked to the fighting at Taftanaz.
NATO could not confirm the type of missile used, but the description fit the Scuds that are in the Syrian military's armoury, the official added, describing the latest launch and similar ones last week as "reckless".
A NATO official said that since the start of December 2012, the alliance had detected at least 15 launches of unguided, short-range ballistic missile inside Syria.
Neither side has gained a clear military advantage in the war pitting mostly Sunni Muslim rebels against security forces dominated by Assad's minority, Shi'ite-linked Alawite sect.
The Observatory also reported fighting between rebels and troops in the Sayyida Zeinab area of Damascus, and air raids were reported in the capital's Maleiha area and eastern suburbs.
Despite some support from Sunni regional powers including Turkey, Saudi Arabia and Qatar, the rebels remain largely disorganised, fragmented and ill-equipped. Poor discipline, looting and insecurity in some insurgent-held areas have also eroded their support from civilians.
Gloom has gripped Damascus for months, as the rebellion edges closer to the capital, but the snowfall offered a rare break from gunfire and shelling echoing from its outskirts.
"We felt a smile that has been missing from our faces for almost two years and we were all just Syrians," said Amin, a resident of central Damascus, speaking on the internet.
"For a few hours our hearts were as pure as the snow."