Russia and China have vetoed a key U.N. resolution on Syria calling for the resignation of President Bashar al-Assad following the massacre of more than 200 people by security forces in the city of Homs.

The other 13 council members, including the U.S., France and Britain, voted in favor of the resolution aimed at stopping the ongoing violence.

Reacting to the vote, France's ambassador to the U.N., Gerard Araud, said: "It is a sad day for the council. It is a sad day for Syria ... History has compounded our shame."

The defeat came despite concerted efforts by western leaders to get security council backing for the resolution censuring the Damascus regime.

Speaking before the vote, Barack Obama called for Assad to step down following the latest bloodshed. The U.S. President said Assad had lost his legitimacy as a ruler and had "no right" to cling to power. He said the regime's policy of terrorizing its people "only indicates its inherent weakness and inevitable collapse".