US, UK warn Syria on chemical weapons threat

The two leaders agreed during a telephone call that "the use - or threat - of chemical weapons was completely unacceptable and would force them to revisit their approach so far," Cameron's Downing Street office said.

"Both said that they wanted to see a credible opposition and hoped that the opposition would use their upcoming meeting in Cairo to show real unity of purpose and coherence in working towards transition."

The White House said Obama conveyed to Cameron his concern about the "increasingly dire" humanitarian situation in Syria, and called for more countries to contribute to humanitarian appeals from the United Nations.

"The two leaders exchanged views on ways the international community can assist those displaced by the conflict," and "apply pressure" on President Bashar al-Assad's regime, it added.

On the ground Syrian forces backed by helicopter gunships and tanks launched a deadly assault on parts of Damascus Wednesday, activists said, as the regime battles to stamp out rebel resistance in the capital.

At least 37 people were reported killed in Damascus alone, a day after a top minister hinted that the embattled regime was ready to discuss President Bashar al-Assad's exit in any talks on ending the brutal 17-month conflict.

The opposition Syrian National Council urged the UN to investigate reports that dozens of people were killed during shelling and in summary executions in Maadamiyat al-Sham, a town west of Damascus.

The SNC described what happened as a "brutal crime" and urged the Arab League to hold an emergency meeting to discuss "war crimes" perpetrated by the regime in Syria."

More than 23,000 people have died since the uprising began in March 2011.