The European Union accused Iran on Thursday of having documents that show how to make nuclear warheads and joined the United States in warning Tehran it faced referral to the U.N. Security Council for possible sanctions.

Iran, meanwhile, suggested it was considering a compromise to reduce tensions.

Britain, in a statement on behalf of the 25-nation bloc, offered new negotiations meant to lessen concerns over Iran's insistence it be in full control of uranium enrichment — a possible pathway to nuclear arms.

"But Iran should not conclude that this window of opportunity will remain open in all circumstances," said a statement read by Peter Jenkins, the chief British delegate to the International Atomic Energy Agency, outside a closed meeting of the 35-nation board.

Diplomats described the statement as a veiled threat of Security Council referral.

"It won't be open for a great deal longer," Jenkins said later when asked how much time Iran had to influence the language of a report to the Security Council.

The final statement was toned down before being delivered to the media.

An earlier version made available to The Associated Press said: "Failure to make progress" on easing international concerns about Iran's nuclear program "will hasten the day when the board decides that a report to the Security Council must be made."

The United States said separately that Iran cannot avoid referral to the Security Council for violating the Nuclear Nonproliferation Treaty but added that Washington and its European allies were delaying such a move to give Tehran a chance to defuse fears it wants to make nuclear arms.

"Iran must understand that the report to the council is required and will be made at a time of this board's choosing," said Gregory L. Schulte, the chief U.S. representative to the Vienna-based IAEA.

But, he said, Washington is ready to wait in hopes that "Iran will reverse course and demonstrate" cooperation both with an IAEA probe of its nuclear activities and an international attempt to re-engage it in talks meant to reduce fears about its intentions.

"One thing is clear, no one wants this dangerous regime to acquire the most deadly of weapons," he later told reporters.

With even traditional allies Russia and China increasing pressure on Tehran, the Iranians are "digging themselves deeper into a hole that threatens to collapse around them," he said.

For months, Iran has relied on Beijing and Moscow to fend off a U.S.-backed push to have it hauled before the Security Council. But the Russians are now working with the Americans and Europeans to push a compromise enrichment plan, and officials recently told AP that China also is moving closer to the Western position.

The main issue is Iran's refusal to give up its right to enrichment, which can be used to generate power but also to make weapons-grade material for nuclear warheads. Iran says it wants only to make fuel, but international concern is growing that the program could be misused.

A plan floated in recent weeks foresees moving any Iranian enrichment plan to Russia. There, in theory, Moscow would supervise the process to make sure enrichment is only to fuel levels.

But Iran insists it wants to master the complete fuel cycle domestically. Iranian Foreign Minister Manouchehr Mottaki told reporters in Tehran on Wednesday that, while his country was willing to resume formal talks with key European powers on its nuclear program, "naturally we aim to have enrichment on Iran's territory."

On Thursday, however, a senior Iranian diplomat appeared to soften his country's stance.

"We are considering it," Mohammed Mehdi Akhounzadeh Basti, the chief Iranian delegate to the IAEA, told the AP when asked about the plan to move Iran's enrichment program to Russia.

Fellow delegate Javad Vaidi said, "We are prepared to follow the path of dialogue with other countries, including the EU-3," referring to France, Germany and Britain, the key EU negotiators.

Jenkins focused on new revelations contained in a report drawn up for the board meeting by IAEA head Mohamed ElBaradei, including a finding showing the Iranians in possession of what appeared to be drawings of the core of an atomic warhead. The agency said last week that Iran obtained detailed designs from the black market run by Abdul Qadeer Khan, the father of Pakistan's nuclear program.

In his statement to the board, also made available to AP, Jenkins said the documents have "no other application than the production of nuclear warheads."

"This reinforces earlier concerns aroused by possible indications of Iranian weaponization activity," he told the board, alluding to a series of findings over the past three years by IAEA experts suggesting that Iran may have experimented with procedures meant to make nuclear weapons.

A separate Iranian statement prepared for the board meeting accused the "U.S. and terrorist groups" of fabricating "false allegations against Iran" in suggesting it was interested in nuclear arms.

It described the find of the warhead documents as a "minor issue" that should not detract from the "tremendous progress achieved by (the) joint cooperation of (the) IAEA and Iran" in clearing up questions about Tehran's nuclear program.