Russia and China have said they have reservations about some of the proposed UN Security Council sanctions to punish Iran for continuing to enrich uranium.
Iran says it has a right to develop peaceful nuclear technology
The Security Council is seeking to expand sanctions imposed in December over concerns Tehran is secretly trying to develop nuclear weapons.
Iran maintains that its nuclear programme is for purely peaceful energy-generation purposes.
Diplomats from Russia and China have balked at the current UN suggestions.
The proposals include an arms embargo, trade restrictions, a travel ban and an extension of an existing list of firms and individuals whose assets would be frozen.
But China fears that financial sanctions would not specifically target the country's nuclear sector but would punish the Iranian people.
"[China's] main difficulty is with the financial and the trade sanctions against Iran because we feel that we should not punish the Iranian people," Chinese ambassador Wang Guangya said after Security Council talks.
Mr Wang said that Russia was unhappy with the specific naming of Iran's Revolutionary Guards on the list of those whose assets would be frozen.
"They feel it is an institution in Iran and you don't penalise an institution," Mr Wang said.
The elite military group of more than 200,000 is controlled directly by the state's supreme leader.
However, US acting representative to the UN, Alejandro Wolff, said that none of the proposals would directly target Iran's civilian population.
"This is an effort to put the focus on the government for being in non-compliance with the Security Council resolutions," he said.
Despite differences between the permanent Security Council members, the BBC's Laura Trevelyan in New York says that diplomats are confident that a resolution can be passed by the end of the week.
Iranian President Mahmoud Ahmadinejad has said his country will not go back on its nuclear programme.
Iran denies Western claims it is secretly trying to build nuclear arms, saying its nuclear programme is for peaceful, energy-producing purposes.
The UN Security Council imposed sanctions on Iran in December, setting a 60-day deadline for it to stop enriching uranium.
But recent report by the International Atomic Energy Agency (IAEA) said that Iran was instead expanding the programme.
Enriched uranium is used as fuel for nuclear reactors, but highly-enriched uranium can be used to make nuclear bombs.