UN Secretary-General Ban Ki-moon has said that there is no prospect of an end to the 21-month conflict in Syria.
Ban told a year-end news conference on Wednesday that the only way to stop the violence was a political settlement, urging the deeply divided UN Security Council to unite and "give a very strong political direction" to the opposing sides.
"Syria began the year in conflict, and ends the year in war," Ban said. "Day by day, the death toll has climbed. Month by month, the regional spillover has grown."
"If nothing is done to change the current dynamic, and to move toward a political solution, the destruction of Syria will be the likely outcome."
Jeffrey Feltman, UN Under-Secretary-General for Political Affairs
For his part, UN Under-Secretary-General for Political Affairs Jeffrey Feltman said in a report to the 15-nation Security Council: "If nothing is done to change the current dynamic, and to move toward a political solution, the destruction of Syria will be the likely outcome".
Feltman's comments came days after Farouq al-Sharaa, Syria's vice-president, conceded that neither side could win the conflict.
Sharaa is the most prominent figure to say in public that Assad will not be able to win the conflict.
Speaking to the Lebanese al-Akhbar newspaper, the vice-president said that he "realise[d] that change is inevitable", but that neither side was in a position to, on its own, say that it was representative of the Syrian people.
UN diplomats and officials, however, say privately that it may be too late for a negotiated solution in a conflict that looks increasingly deadlocked.
The UN concerns come as Syrian rebels claim to have captured at least six towns in the central province of Hama.
Qassem Saadeddine, a member of the rebel military command, said most of the rural western part of Hama province was under the control of the rebels.
"The battle is ongoing. We have freed many areas [of Hama] and we are studying plans to liberate more," Saadeddine told Reuters by Skype from inside Syria. "Three-quarters of western rural Hama is under our control."
He said the towns taken by rebels included Latamneh, Helfaya, Kfar Naboudah, Hasraya, Tibat al-Imn and Kfar Zita, and that fighting had also broken out in the city of Hama itself.
The Syrian Observatory for Human Rights, an opposition-linked group which monitors violence across the country, said several of the Hama towns overrun by rebels came under bombardment on Wednesday.
In Damascus, rebels who seized control of the Yarmouk Palestinian refugee camp on Monday declared the neighbourhood a "liberated area" on Wednesday and handed it over to Palestinian control.
The Syrian opposition's recent success is one of a few significant military and diplomatic gains in recent weeks.
They have captured several army installations across Syria and secured formal recognition from Western and Arab states for a new coalition.
The capture of large parts of Hama province could now give the rebels effective control of a stretch of territory from the northern Turkish border 180km to the north.