UN special envoy Kofi Annan will seek Russian President Vladimir Putin's help in ending the conflict in Syria during a visit to Moscow starting on Monday, with pressure mounting on Russia, which has already vetoed two resolutions sanctioning the Syrian government, to stop backing Syrian leader Bashar al-Assad.

Sergey Lavrov, the Russian foreign minister, gave no indication that Russia would change its position on Syria in a speech on Monday.

He says western countries should not threaten to use force in negotiations to extend the UN 's observer mission, which is due to run out in a few days.

He said UN observers are being used to blackmail Russia into backing a new resolution.