Britain, the US and France are seeking Russian support for a new United Nations security council resolution to endorse Arab demands that Syria's president, Bashar al-Assad, step down.

In a fresh twist to international diplomatic attempts to deal with the Syrian crisis, western countries have, in effect, abandoned attempts to impose UN sanctions on the Assad regime and are hoping for a new consensus for a political solution to the bloody 10-month impasse.

The basis for the new approach is the Arab League plan published on Sunday that called on Syria's president to hand over powers to his deputy and set up a national unity government with the opposition. Syria lambasted the plan as "flagrant interference" in its internal affairs and accused Arab states of attempting to "internationalise" the crisis.

The secretary general of the Arab League, Nabil al-Arabi, and Hamed bin Jassem AlThani, the Qatari prime minister, are planning to brief the UN secretary general, Ban Ki-moon, in New York in the next few days, Arabi's deputy, Ahmed ben Helli, revealed in Cairo on Tuesday.

If Russia is supportive, the hope is to pass a UN resolution early next month forcing Assad to confront solid international opposition for the first time since the unprecedented uprising began last March, diplomatic sources said.

Russia has so far backed Assad and opposed punitive action or even verbal condemnation in part because it is angry at the way the UN was used to mandate the Nato intervention it opposed in Libya .

But it formally welcomed the league decision, and western diplomats say they believe it will now be hard for Moscow to veto an explicit Arab call for the Syrian leader to step down.

On Monday a Kremlin envoy appeared to rebuke Assad for failing to promote a peaceful solution, though the foreign ministry was quick to insist that Moscow's position was unchanged.

China, which is also one of the permanent five veto-wielding members of the security council, has blocked anti-Syrian action but is thought likely to shift position if Russia does. The US has criticised Russia for supplying weapons to Syria, which received a shipment of Russian ammunition this month and has signed a $550m (£352m) deal for 26 Yak-130 jet trainer aircraft. The Syrian port of Tartous on the Mediterranean coast is an important base for the Russian navy.

Western governments also point out that the Arab League plan is a Syrian version of the long-negotiated departure of the Yemeni president, Ali Abdullah Saleh, which was brokered by western and Arab states and accepted by Russia.

But Syria's foreign minister, Walid al-Muallem, expressed confidence that Russia would stand by its long-standing ally. "Our relations with Russia have deep roots," he told reporters in Damascus. "Russia cannot welcome foreign intervention in Syria. That is too much."

Syria no longer wanted "Arab solutions to the crisis", Muallem insisted. "Heading to the security council will be the third stage in their plan, and the only thing left is the last step of internationalisation. They can head to New York or to the moon. So long as we are not paying for their tickets it is none of our concern."

Qatar's Sheikh Hamad, the leading anti-Assad hawk, had warned the league would go to the UN if the Damascus government did not implement the initiative, which European governments have welcomed as a potential "game-changer". The emerging diplomatic initiative overshadowed continuing doubts about the future of the troubled league monitoring mission in Syria, whose Saudi and other Gulf members have now been formally withdrawn.

But it is understood that they may be replaced by observers from Muslim countries such as Indonesia and Turkey. Syria agreed to extend its mandate by a further month. The Damascus government has seen the observers as a useful way of being seen to co-operate, though their mission has been heavily criticised by the Syrian opposition for failing to halt Assad's crackdown.

The opposition Syrian Revolution General Commission said 15 people had been killed across the country on Tuesday. Other organisations gave higher figures but none could be confirmed.

In another development an officer in Syrian air force intelligence, the most feared of the regime's security bodies, announced live on an opposition TV channel that he was defecting.

Britain is openly urging the Arab League to "ask the UN security council to pass an appropriate resolution that will assist towards saving lives in Syria", William Hague, the foreign secretary, said after talks with his Australian counterpart.