US and Russian officials have given their commitment to a political solution for the deepening Syrian conflict, a United Nations envoy said on Sunday, but Moscow dismissed speculation it was preparing for President Bashar al-Assad's exit.

With rebels now fighting on the doorsteps of Damascus, Assad's forces kept up their now daily artillery strikes and air raids on eastern suburbs as well as some rebel-held districts on the capital's outskirts.

UN special envoy Lakhdar Brahimi met the US and Russian deputy foreign ministers in Geneva for the second session of tripartite talks in less than a week, apparently in response to rising violence that now threatens to engulf Damascus.

"All three parties reaffirmed their common assessment that the situation in Syria was bad and getting worse," a statement from Brahimi said. "They stressed that a political process to end the crisis in Syria was necessary and still possible."

Notwithstanding his comments, commitment to a political process could be at risk. Western officials were among those who helped rebels to create a unified chain of command at the weekend, and Washington is expected to recognize the opposition as the sole representative of the Syrian people next week.

Russia, Syria's main arms supplier, insisted its position on Assad had not softened and it was not negotiating on the future of the president, whose family has ruled Syria for 42 years.

"We are not holding any talks on the fate of Assad," Foreign Minister Sergei Lavrov was quoted as saying by Itar-Tass news agency in Moscow. "All attempts to present the situation rather differently are shady."

Brahimi said the talks aimed to find a solution based on last June's Geneva Declaration, which called for a transitional government. This proposal originally foundered over different interpretations of that transition. Washington said Assad could not play a role but Russia insisted that his fate should not be decided outside Syria.

Reuters