* U.N. mandate expires at midnight Friday
* Pakistan resolution calls for 45-day rollover of mandate
* British extension calls on Syria to halt heavy-weapons use
UNITED NATIONS, July 20 (Reuters) - U.N. Security Council envoys debated competing resolutions aimed at extending the U.N. monitoring mission in Syria on Friday, but Russia said it would veto a British proposal and a rival measure by Pakistan lacked clear support.
The Security Council had been expected to vote at around 10 a.m. (1400 GMT) on one or both draft resolutions, but differences among the 15 member states led to further consultations first, envoys said.
Both resolutions seek to extend the U.N. observer mission in Syria before its mandate expires at midnight Friday EDT (0400 GMT on Saturday).
But it remained unclear whether either resolution had enough backing to pass. It would need at least nine votes and no vetoes by permanent members United States, Britain, France, Russia or China.
Russia and China on Thursday vetoed a Western-backed resolution that would have extended the mandate, saying they could not accept the measure's call for sanctions against the government of Syrian President Bashar al-Assad if it fails to rein in its campaign against a widening rebellion.
Britain has proposed an alternate version which would extend the observer mission mandate by 30 days, but Russia's U.N. envoy said on Friday Moscow would veto that as well because it specifically called on Damascus to halt the use of heavy weapons.
"We're going to vote against, we told them yesterday," Russian ambassador Vitaly Churkin told reporters when asked if Russia would veto the British draft resolution.
China's U.N. Ambassador Li Baodong said Beijing supported Pakistan's proposal for a technical rollover of the mission for 45 days, which Russia also backs.
"We're looking for a consensus, there's still a chance," Li said when asked if China, too, contemplated another veto of the British proposal.
With the council in disarray, Pakistan asked for further consultations on Friday before a vote is held, diplomats said.
U.S. Ambassador to the United Nations Susan Rice said on Thursday that the United States would "probably" support the British resolution and described the Pakistani resolution as a simple technical rollover. "We have been clear, we are not supporting a technical rollover," she told reporters.
Russia and China have thus far vetoed three different resolutions designed to pressure Assad and halt the 16-month conflict that has killed thousands. (Reporting by Michelle Nichols; Writing by Andrew Quinn; Editing by Jackie Frank)