The U.N. Security Council is expected to vote this weekend on a resolution imposing a third round of sanctions on Iran over its nuclear program, Western ambassadors said on Thursday.

We thought it was perfectly reasonable to (finalize the resolution) tomorrow and to vote on this important issue on Saturday, said U.S. Ambassador Zalmay Khalilzad.

Washington had been pushing for a vote on Friday.

His British counterpart, John Sawers, said the vote would probably be on Saturday but did not rule out another delay.

Had we put it to a vote today the resolution would clearly have passed but we want to have as much support as is possible, Sawers said. One delegation has suggested some small amendments which we're reflecting upon.

Diplomats said he was referring to a proposal from Vietnam, which has expressed concerns about the resolution drafted by Britain, France and Germany.

Diplomats also said South Africa, which has said it wanted to delay the sanctions vote, had requested a meeting with the European co-sponsors on Friday to discuss the resolution.

A third country which has said it might not be able to support the resolution, Indonesia, indicated it might have to abstain or even vote against it.

We are yet to be convinced that more sanctions is the reasonable way to go at this time, Indonesian Ambassador Marty Natalegawa said. Indonesia voted for the last sanctions resolution against Iran, which was adopted in March 2007.

Libya's ambassador indicated on Monday that he would probably vote against the resolution.

Even without the votes of Libya, Vietnam, Indonesia and South Africa, the resolution has 11 out of 15 council members -- including all the permanent members -- ready to support it so seems certain to be approved.