Tehran has already been hit with three rounds of UN blacklistings, travel bans and asset freezes aimed at individuals and companies involved in its nuclear and missile programs.
Western powers pushing for new sanctions against Iran over its nuclear program face a battle with Russia and China, which may only be persuaded to support steps that are more symbolic than painful.
For weeks, officials from the United States, Britain, France and Germany have been discussing the kinds of punitive measures they should include in a draft UN Security Council resolution they hope to show to Russia and China as early as next week, diplomats said on condition of anonymity.
"It's time to start haggling with the Russians and Chinese so we can get a sanctions text to the Security Council in the near future," one Western diplomat said. "We believe we can get their support, though it will come at a price."
Another senior diplomat predicted the final result would be a "symbolic" tightening of sanctions against Tehran.
"Specific sanctions measures aside, the unity of the six will send a strong signal to Iran," the second diplomat said.
Tehran has already been hit with three rounds of UN blacklistings, travel bans and asset freezes aimed at individuals and companies involved in its nuclear and missile programs. But Iran continues to reject the Security Council's demands for a halt to its nuclear enrichment program.
The UN nuclear watchdog in Vienna suggested in its latest report on Iran that the Islamic Republic was actively pursuing a nuclear weapons capability.
Tehran, which says its nuclear program is for civilian and medical use and rejects Western allegations Iran wants atomic weapons, dismissed the watchdog's report as misleading, unbalanced and incomplete.
Russia and China, which both have veto power on the Security Council, have close business ties with Iran. While they supported the three rounds of UN sanctions against Tehran, they fought hard to dilute all three before the votes.
But there are signs China would reluctantly vote for new sanctions. Analysts say it would probably join Russia if Moscow decided to support a fourth sanctions resolution.
The Western powers hoped the five veto-wielding permanent Security Council members and Germany could agree on a draft resolution by the end of February, which the six could submit to the full council to be voted on by the end of March.
But diplomats said negotiations with the Russians and Chinese would take time and a vote in the full council was unlikely next month. US secretary of state Hillary Clinton said on Wednesday it might not happen until late April.
"We hope that in the next 30-60 days we'll see a sanctions resolution emerge in New York," Clinton said, adding there may be "bilateral or multilateral sanctions on top of whatever may be the result of the Security Council deliberations."
European Union diplomats have said the 27-nation bloc hoped to follow up any new UN sanctions with its own tougher measures aimed at curtailing EU business with Iran.
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