Explosions echoed over Damascus today as Syrian troops clashed with rebels in some of the heaviest fighting yet in the capital in the 15-month uprising against President Bashar Assad.

Syrian troops shelled two rebel-held neighbourhoods in the central city of Homs today, activists said.

A deadline for a United Nations-brokered ceasefire passed today without reports of major violence, opposition activists said, hours after Syria promised to observe a halt in fighting.

US secretary of state Hillary Clinton vowed today to pressure Russia over the Syria crisis, saying its refusal to support action by the United Nations Security Council was keeping President Bashar Assad in power.

A UN-brokered plan to stop the bloodshed in Syria has effectively collapsed after President Bashar Assad's government raised new, last-minute demands that the country's largest rebel group swiftly rejected.