The United States, which accuses Iran of seeking to build nuclear weapons, reiterated that Iran may be referred to the UN Security Council, while Russia adopted a more conciliatory tone by saying dialogue was still the only way forward.
White House spokesman Scott McClellan told reporters that the international community had already warned Iran that "the next step would be a referral" to the Security Council.
He said Iran must maintain a total suspension of activities linked to uranium enrichment, which produces fuel for nuclear power reactors but can also be used to make atomic bombs.
"The international community has growing concerns about the regime's pursuit of nuclear weapons under the guise of a civilian nuclear program," Mr McClellan said.
"The comments and actions of the regime only further isolate Iran from the rest of the international community," he said.
Wolfgang Schussel, Chancellor of Austria, which now holds the rotating EU presidency, said the possibility of sanctions existed, “but should be a last resort”.
Russian Foreign Minister Sergei Ivanov, whose country has a veto in the Security Council, said the issue must be resolved by political means and under the International Atomic Energy Agency (IAEA)'s auspices.
"This problem must be resolved primarily within the political and diplomatic framework, and on the current stage, within the IAEA framework," Mr Ivanov said on Russian television.
Iran's announcement of a resumption coincided with the suspension of talks between Iran and Russia aimed at seeking a compromise over Iranian uranium enrichment, a key phase in the fuel cycle.
"Today, under the supervision of the agency, research activities will resume," Iranian government spokesman Gholam Hossein Elham said on Monday, referring to the IAEA.
However, by late Monday there had been no announcement in Tehran of an actual resumption.
Iran had asked the IAEA to have inspectors ready to witness the removal of UN-supervised seals at its research centres, although as the suspension was voluntary, IAEA inspectors were not required to supervise the procedure.
But Hossein Entezami, spokesman for the National Security Council which is in charge of the nuclear file said Iran was counting on the UN watchdog agency's cooperation to resume its research.
"I hope that the agency (IAEA) will do the necessary so that the research activities resume today," he told news agency AFP.
But IAEA chief Mohamed ElBaradei said he was "losing patience" with what he called Iran's lack of transparency.
He told Britain’s Sky News television in an interview recorded before Iran's announcement that he still needed clarification about Iran's actions.
"There are still a number of important issues where I have not been able to make progress and I still need very much Iran's transparency and Iran's active cooperation," he said.
Europe has warned that restarting fuel work, which would end a two-year suspension, would jeopardise any resumption of wider talks on ending the crisis with the West over Iran's nuclear activities.
Germany, which along with Britain and France makes up the EU troika leading negotiations with Iran, warned that the decision "cannot remain without consequence".
"This marks a breach of Tehran's commitments," Foreign Minister Frank-Walter Steinmeier said.
His French counterpart, Philippe Douste-Blazy, urged Iran "to immediately and unconditionally reverse its decision".
Defiance
But Iran's supreme leader Ayatollah Ali Khamenei voiced defiance.
"The Islamic Republic of Iran will not give up its undeniable rights to peaceful nuclear technology, which has been achieved by the talented youth of the country," Mr Khamenei said in Tehran.
"The ones who are invoking sanctions have sanctioned Iran whenever they could ... such sanctions have no effect," the leader said.
Iran has been trying to draw a distinction between research into the fuel cycle and actual production of enriched uranium, which can be used as fuel in civil reactors or, in highly enriched form, as the explosive core of an atom bomb.
Monday's announcement came after talks between Russia and Iran on a proposed compromise to end the row over uranium enrichment broke off without result on Sunday, although they are to resume in a month.
Russian officials are proposing that Iran carry out uranium enrichment on Russian territory to allay Western fears that the technology could allow Iran to produce a nuclear bomb.