"We're chipping away at any remaining differences that might exist," McCormack said.

The governments -- the five permanent UN Security Council members plus Germany, known as the P5+1 grouping -- handed the sanctions issue to their UN ambassadors on Monday in hopes of drawing up a draft resolution this week.

But the talks have since bogged down, with a senior US official saying one problem was Chinese objections to elements in the sanctions proposal.

"We're working through some stuff with them," the official, speaking on condition of anonymity, said of the Chinese.

The Security Council adopted a first series of sanctions against Iran in December after Tehran refused to freeze a uranium enrichment program that could produce fuel for nuclear power stations but also material to make atomic bombs.

That resolution set a 60-day deadline for Iran to suspend its program. When the deadline passed in late February, the UN atomic watchdog reported that Iran had actually accelerated its enrichment activities.

P5+1 diplomats have described the second sanctions proposal as an "incremental" ratcheting up of the December measures, which included a ban on the sale of nuclear and ballistic missile-related materials to Iran and a freeze on assets of individuals or companies involved in the programs.

The new proposal would include a travel ban on nuclear officials, an arms embargo and trade and financial sanctions, including possible restrictions on export credits to Iran, diplomats said.

Negotiations on the December resolution dragged on for months, mainly due to efforts by Russia, which has extensive energy and other dealings with Iran, to water down the measures.

McCormack insisted Russia was not the problem in the current round of talks.

"I want to dispel this notion that's out there -- it may be an echo of past discussions -- but certainly in this round we're working very cooperatively with the Russian government," he said.

The US spokesman expressed optimism that despite ongoing differences the new sanctions resolution would be passed quickly, though he declined to set any timeframe.

"Make no mistake, we are going to get a resolution," he said.

"Everybody is committed to getting a second resolution as soon as we possibly can."

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