But US Undersecretary of State John Bolton, a leading arms control official, told a news conference that Washington and its European allies would keep up pressure on Tehran to come clean on its uranium enrichment program in other ways.

"We are absolutely determined not to reduce the pressure on Iran," he said.

"We think the pressure they have been under has been critical to their revealing the pieces about their nuclear program that they have already revealed."

Washington, which accuses Iran of trying to build an atomic bomb, has said in the past it felt the Vienna-based International Atomic Energy Agencyshould refer the country to the United Nations Security Council for possible sanctions.

Diplomats however say the United States would almost certainly not have backing for sanctions against Iran when the IAEA board begins a week-long meeting on Monday because many European nations still want to give Iran a chance to cooperate.

The head of the UN nuclear watchdog, Mohamed ElBaradei, said Tuesday he believed Tehran was cooperating better with global non-proliferation efforts and the situation was evolving "in the right direction".

Last October Tehran agreed to European demands for more intrusive inspections of Iranian nuclear sites and a total suspension of activities related to the enrichment of uranium.

ElBaradei would not say what measures the IAEA board would take when it meets next week.

The United States meanwhile insists Tehran continues to hide its weapons program.

"We see a continuing pattern of deception and concealment by the Iranians," said Bolton, who is considered one of Washington's leading "hawks".

"We think the Iranians are still trying to conceal a clandestine weapons program and this is why it remains a grave concern to us."

Iran maintains its uranium enrichment program is intended to generate electricity and is not to be used to build a nuclear bomb.

Bolton said Europe and the United States shared the same goal -- the elimination of Iran's weapons of mass destruction program -- even though they had a different strategy for how to achieve this objective.

He added Washington would seek to refer Libya to the Security Council when the IAEA board meets next week for having hidden its nuclear weapons program in the past.

But Bolton said this would be a formality since the country in December pledged to abandon its weapons of mass destruction program and the resolution would praise this move.

"In the case of Libya they have made a decision that the pursuit of weapons of mass destruction was not enhancing their national security," he said.

"In fact the pursuit of weapons of mass destruction was making them less secure."

He added the approach taken by Libya was in "dramatic contrast" to that taken by Iran and North Korea, which Washington also accuses of pursuing a hidden weapons program.

Bolton was speaking at the start of a two-day meeting in Lisbon of 14 nations which are part of the Proliferation Security Initiative, which aims to stop shipments of weapons of mass destruction.

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