Another of the sceptics, Indonesia, indicated it might abstain or even vote against the resolution.
"We are yet to be convinced that more sanctions is the reasonable way to go at this time," its ambassador, Marty Natalegawa, said. Indonesia voted for the last sanctions resolution against Iran that was adopted in March 2007.
Libya's ambassador, Giadallah Ettalhi, 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 it seems certain to be approved.
The draft resolution calls for more travel and financial restrictions on named Iranian individuals and companies and makes some restrictions mandatory. Two earlier sanctions rounds were approved unanimously in December 2006 and March 2007.
Iranian Foreign Minister Manouchehr Mottaki wrote a letter to the council arguing that sanctions against Iran lacked any legal basis and only undermined the council's credibility.
Mottaki said a recent report from the U.N. nuclear watchdog citing progress in its investigation of Iran's past nuclear activities proved the Security Council had no right to punish Tehran for continuing its enrichment program.
"All the so-called justifications and flawed foundations for the U.N. Security Council's action on this issue are vanished and it shows that the resolutions ... lack any legal and technical justifications," Mottaki wrote.
The letter, addressed to Panamanian Ambassador Ricardo Alberto Arias, the council's current president, was dated February 27 and circulated to reporters on Thursday.
"Naturally the continuation of this trend (of sanctions) would undermine the credibility of the Security Council and ... weaken the integrity and position of the IAEA," Mottaki wrote.
The report of the International Atomic Energy Agency (IAEA) was mixed, saying Iran had not responded adequately to Western intelligence allegations of work linked to making atomic bombs but had clarified issues related to other past nuclear work.
Western countries do not believe the IAEA report vindicated Iran, as Mottaki argues. They say it raises serious concerns.
Iran's U.N. ambassador, Mohammad Khazaee, said Tehran had no intention of curtailing cooperation with the IAEA even if the resolution is passed.
But, he said, Iran will keep enriching uranium. "Suspension I think is out of the question," he said.
