Russia's deputy foreign minister said Friday that Moscow could not support the latest UN Security Council draft resolution on Syria in its current form, the Interfax news agency reported.

"Some of our concerns and the concerns of those who think the same as us have been taken into consideration but all the same this is not enough for us to be able to support it in this form," deputy minister Gennady Gatilov said.

"We still have a whole number of concerns over the content of this text and we will be ready to continue consultations on the draft resolution," Gatilov said.

"We are ready to continue work on modifying it, taking into consideration and based on our principled positions."

He spoke after a senior US State Department official said that Washington was "cautiously optimistic" about winning Russia's support for the draft.

The latest draft does not explicitly call on Syrian President Bashar al-Assad to step down or mention an arms embargo or sanctions, though it "fully supports" an Arab League plan to facilitate a democratic transition.

The latest attempt at consensus emerged amid an impasse in the UN Security Council, with Russia leading the opposition to a tougher draft resolution authored by Western powers and the Arab League.

The new draft "fully supports" the January 22 Arab League request that Assad transfer power to a deputy and a government of national unity within two months but does not call on him to step down, according to a copy obtained by AFP.

Moscow, a long-standing Assad ally and one of his top arms suppliers, has declared that the UN body does not have authority to impose a resolution that calls for regime change in Syria, a position supported by China.