MOSCOW: Russia on Monday defiantly slammed the West for using "blackmail" over a new UN Security Council resolution on Syria and ridiculed the idea it could convince President Bashar al-Assad to step down.

In combative comments that underlined the huge gulf between Russia and its Western partners, Foreign Minister Sergei Lavrov accused the West of making an extension of the UN observers mandate in Syria conditional on Russia agreeing sanctions against Damascus.

He also bluntly shot down any lingering hopes that Russia could bring about an end to the crisis by persuading Assad to step down, saying the Syrian leader would never quit as he still retained popular support.

The splits between Russia and Western powers risk being played out in a new standoff at the UN Security Council, where the West wants Moscow to back a resolution that would impose new sanctions against Syria.

"To our great regret, we are witnessing elements of blackmail," Lavrov told reporters hours ahead of talks with UN-Arab League Syria envoy Kofi Annan in Moscow.

"We are being told to either agree to the approval of a resolution that includes Chapter 7 (that provides for possible sanctions), or we refuse to extend the mandate of the observer mission.

"We view this as a completely counter-productive and dangerous approach," Lavrov said.

The looming diplomatic clash at the United Nations comes ahead of a Friday deadline for the end of the current three-month mandate of the UN observer mission in Syria.

Annan, who is due to have a working dinner with Lavrov later Monday before meeting President Vladimir Putin on Tuesday, is visiting Moscow with his peace plan to end the violence disintegrating amid the escalating killings.

But Lavrov angrily rejected suggestions that Russia and its chief diplomatic ally China had taken the side of the Assad regime, saying Moscow was pursuing an even-handed approach in contrast to the West's support of the opposition.

"I will repeat -- we are not supporting Bashar al-Assad. We are supporting what everyone else is -- the peace plan of Kofi Annan," said Lavrov, adding it was "particularly wrong" to blame Russia and China for the bloodshed.

The opposition and Western states have urged Moscow to help end the conflict by taking a tougher line against the Assad regime, whose close ties with Moscow date back to the alliance between late president Hafez al-Assad and the USSR.

But Lavrov stressed that it was "unrealistic" for Western powers to expect Russia to convince Assad to step down simply because Moscow is a long-standing ally of Damascus.

"We hear comments like the 'key to a Syrian solution is to be found in Moscow'. Then it is explained to us, when we ask about this, that it means that we (Russia) have to convince Assad to step down of his own accord," said Lavrov.

But he said: "It is simply unrealistic. And it is not a question of our inclinations, our sympathies or our antipathies.

"He (Assad) will not leave power. And this is not because we are protecting him but because there is a very significant part of the Syrian population behind him."

Lavrov appeared to admit that Moscow was equally unable to press the Syrian opposition, which last week made rare visits to the Russian foreign ministry but failed to narrow differences with Russia.

"So far we have not succeeded in getting through to them (the opposition) the necessity of renouncing radical positions. They are continuing to say that a revolution is in progress," Lavrov complained.

The head of Syria National Council umbrella opposition group described having a tense meeting of more than three hours with Lavrov on July 11 in which "we told them that the Syrians are being killed by Russian weapons."

"Their answer is that they had stopped shipments of arms," Abdel Basset Sayda told CNN. AGENCIES