Russia and China on Thursday vetoed a UN Security Council resolution that would threaten sanctions against Syria's President Bashar al-Assad if he does not end the use of heavy weapons.

The third Russia-China veto in nine months opened up an acrimonious battle at the 15-nation council over who is to blame for the world powers' failure to get international action to halt the Syria conflict.

There were 11 votes in favor, with Russia and China voting against and with Pakistan and South Africa abstaining. As two of the five permanent members of the council, Russia or China can veto any resolution.

Amid growing doubts over the future of the peace mission of UN-Arab League envoy Kofi Annan, the United States said it would now act outside of the UN body to confront Assad. Russia said the West wanted "military intervention".

"We will intensify our work with a diverse range of partners outside the Security Council to bring pressure to bear on the Assad regime and to deliver assistance to those in need," US ambassador Susan Rice said.

"The Security Council has failed utterly in its most important task on the agenda this year," she added, while warning of reports that Assad's government could use chemical weapons in the battle against the opposition.

"The United Kingdom is appalled at the veto of Russia and China," said Britain's UN envoy Mark Lyall Grant, whose country -- another permanent council member -- took the lead in writing the resolution.

"The effect of their actions is to protect a brutal regime. They have chosen to put their national interests ahead of the lives of millions of Syrians," Lyall Grant told the council.

"It is clear that Russia only aims to give more time to the Syrian regime to crush the opposition," said France's envoy Gerard Araud.

"Refusing Annan the means of pressure that he asked for is to threaten his mission," Araud told the Council.

The British text, backed by the United States, France, Germany and Portugal, threatened non-military sanctions under Chapter VII of the UN Charter if Assad does not withdraw heavy weapons from Syrian cities in 10 days.

However the sanctions were only a threat and would have needed a new resolution, western envoys stressed.

Russia accused Western nations of seeking to use the proposed resolution to justify military intervention in Syria.

The resolution sought to "open the path to the pressure of sanctions and further to external military involvement in Syrian domestic affairs," Russia's UN ambassador Vitaly Churkin said after his veto.

Churkin said the West had sought to "fan the flames of extremists, including terrorist groups."

China's ambassador Li Baodong said Western nations had been "arrogant and rigid" in negotiations on the resolution.

The sanctions proposal was added to a resolution on renewing the UN Supervision Mission in Syria (UNSMIS) whose 90-day mandate ends on Friday.

Without a resolution, the UN may have to hurriedly withdraw the nearly 300 unarmed observers now in Damascus.

The UNSMIS operation has been suspended since June 16 because of the mounting violence.

Russia had said it could not accept sanctions or action under Chapter VII.

It drew up a counter-resolution which would just renew the UNSMIS mandate but withdrew the text because it became clear that it could not get the nine votes needed to pass.

The five Western countries along with Colombia and Morocco had indicated they would abstain on the Russia text, assuring its failure.

India's ambassador Hardeep Singh Puri said it was "regrettable" that the council had not sent a united message to the Syrian leadership to back Annan. He said council powers had acted on their "national interests".

Pakistan, which abstained, said that urgent action must be taken to make sure UNSMIS stays in Syria.

More than 17,000 people have been killed since an uprising against Assad began 16 months ago, activists say. The Security Council faces growing criticism over its failure to take action.

UN chief Ban Ki-moon and international envoy Annan had called on the Council to unite and take strong action on Syria in the hours ahead of the vote.

Ban said there was an "extreme urgency" for action to make government and opposition forces halt the violence. Annan said the council must take "decisive" action on the conflict.