MANAUS, Brazil (Reuters) - Washington has intelligence suggesting Iran is working on the technology to deliver a nuclear warhead on a missile, Secretary of State Colin Powell said Wednesday in a claim that could increase fears about Tehran's nuclear activities.

The top U.S. diplomat, who has pressed in vain for Iran to be referred to the U.N. Security Council because of its suspected nuclear weapons programs, said the disclosure should concern the international community.

"I have seen some information that would suggest they have been actively working on delivery systems," Powell told reporters during a brief stop in Brazil on his way to an international meeting in Chile.

"I'm talking about what one does with a warhead," Powell said. "We are talking about information that says they not only have (the) missiles but information that suggests they are working hard about how to put the two together."

His remarks came on the same day an Iranian exiled opposition group also made several claims about the extent of the Islamic Republic's nuclear programs.

And his comments also followed an accord this week European powers hammered out for Iran to suspend some nuclear activities that could undermine the U.S. push for a U.N. referral and possible international sanctions.

The exile group, which has been accurate before, said Iran obtained weapons-grade uranium and a nuclear bomb design from a Pakistani scientist who has admitted to selling nuclear secrets abroad.

Powell said he did not know if the claims about the scientist were true but noted U.S. intelligence matched the group's charge about the warhead design.

"I have seen intelligence which would corroborate what this dissident group has said (about weaponizing a warhead) and it should be of concern to all parties," he said.

POLICY CRISIS

President Bush has vowed Iran, a country he bracketed in an "axis of evil" with pre-war Iraq and North Korea, will not acquire a nuclear bomb. Political analysts say Iran could be Bush's biggest foreign policy crisis in his second term.

U.S. officials have estimated Iran is three to five years from developing a nuclear weapon.

Iran says its nuclear programs are for power generation and other nations have been skeptical about U.S. claims following its flawed intelligence over Iraq's weapons programs.

Tuesday, the U.N. nuclear watchdog, summarizing its two-year probe of Iran's nuclear activities, said Tehran had not diverted declared nuclear materials to a weapons program.

Joseph Cirincione, director of the Carnegie Endowment for International Peace's Non-Proliferation Project, said arms control experts have long assumed Iran was pursuing the technological challenge of mating nuclear weapons and missiles.

"Powell seems to be hinting at some new information that he has indicating that they are pushing to actually make this warhead design (work) with their existing missiles," Cirincione said. It took considerable expertise to shrink a nuclear bomb to fit on a missile with a one-ton payload and to make it sturdy enough to survive rocket launch and reentry, he said.

The outgoing secretary of state may have a rare chance to raise directly with Iranians his concerns over the nuclear programs.

Powell held out the possibility he could talk with the Iranian delegation informally during an international conference this month in Egypt about Iraq.

"Whether there is a social opportunity or not remains to be seen," he said.

Powell gave a guarded welcome to a deal Britain, France and Germany struck this week for Iran to suspend uranium enrichment activities.

Uranium enrichment can be used in power generation or to make a bomb.

Powell called the deal "progress" but said Washington would watch if the deal holds up "with necessary, deserved caution."