EU foreign policy chief Javier Solana earlier Monday held what a spokesman described as "inconclusive" talks with Iran's top nuclear negotiator, Saeed Jalili, after Tehran missed a deadline to respond to the compromise offer.
State Department spokesman Gonzago Gallegos said Jalili told Solana that Tehran would provide a written response to the proposed trade and economic incentives on Tuesday.
Senior diplomats from the United States, Britain, France, Germany, Russia and China discussed the latest developments by telephone Monday and agreed to push for new action against Iran if it continued holding out.
"We agreed that in the absence of a positive response, we have no choice but to pursue further measures," Gallegos said.
"We are disappointed that we have not yet received a response from Iran as requested in Geneva on July 19," he said, referring to a meeting of senior officials from the six powers with Iran at which the incentive offer was made.
The United Nations has imposed three sets of sanctions against Iran over the nuclear dispute and is mulling a fourth round of measures.
Tehran has steadfastly refused to suspend its uranium enrichment activities, which it says are aimed only at producing fuel for nuclear power production.
The United States and its allies fear the program is a cover for developing nuclear weapons.
A spokesman for Solana said in Brussels that further contacts between the EU and Iran "are not ruled out in the coming days".
But he did not mention any pledge by Jalili to provide a written response on Tuesday to the big powers' proposal.
Iranian state-run television meanwhile reported that in the Solana-Jalili telephone conversation, "both sides agreed to continue talks."
"They also emphasized that preserving this path (talks) needs a positive and constructive atmosphere," the television report said without elaborating.
The United States had demanded that Iran meet a deadline of last weekend to respond to an international package of incentives aimed at persuading Tehran to freeze uranium enrichment amid warnings of new sanctions.
But an EU diplomat said on Friday: "If it's in 16 days instead of 14 it's not a problem. We are not obsessed with a date."
Iran had earlier dismissed talk of the deadline as "media speculation."
Solana presented an offer of economic and trade incentives in mid-June, while Iran has put forward its own proposal, an all-embracing package of suggestions to resolve the problems of the world, including the nuclear issue.
Monday's talks were the first contact between the two sides since Geneva.
Along with its regional ally Israel, Washington has warned that the option of military action against Iran remains open if Tehran sticks to its defiant line.
Amid the continued tensions, Iran announced on Monday that it had successfully test-fired an anti-ship missile with a range of 300 kilometers (180 miles) that it had developed with homegrown technology.
"No enemy vessels would be able to escape it within a 300-kilometer radius from the borders of Iran," the commander of the elite Revolutionary Guards, General Mohammad Ali Jafari, said on state television.
Iran has in recent months frequently boasted of developing new weapons and military hardware but the claims have often met with skepticism from Western defense analysts.
World oil prices -- which have been driven to record levels by worries over a possible military strike on OPEC's number two producer -- were mixed on Monday.
New York's main contract, light sweet crude for September delivery, fell 16 cents to 124.94 dollars a barrel in electronic trading, reversing earlier gains.