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OMAHA, Neb., Dec 5 (Reuters) - U.S. President George W. Bush told Iran on Wednesday to "come clean" about its nuclear activities or else face isolation by the international community.

Bush issued the stern warning as he sought to keep pressure on Iran despite a new intelligence report that Tehran halted its nuclear arms program four years ago, contradicting his earlier assertions that it was building an atom bomb.

Standing on the airport tarmac upon arrival in Omaha, Bush insisted that Iran has not fully accounted for its nuclear activities as it continues to defy demands to halt uranium enrichment.

"It is clear from the latest NIE (National Intelligence Estimate) that the Iranian government has more to explain about its nuclear intentions and past actions, especially the covert nuclear weapons program pursued into the fall of 2003 which the Iranian regime has yet to acknowledge," Bush said.

Bush spoke a day after he insisted that Iran remains dangerous and urged continued international pressure.

The White House said on Wednesday that the United States would press ahead with its campaign for a third round of U.N. sanctions on Iran.

The shift in the intelligence community's thinking on Iran, however, was expected to stiffen resistance to further punitive measures.

Bush said Britain, France, Germany and Russia agree that the Iranian nuclear issue continues to be a problem.

"The Iranians have a strategic choice to make, they can come clean with the international community about the scope of their nuclear activities and fully accept the longstanding offer to suspend their enrichment program and come to the table and negotiate, or they can continue on a path of isolation that is not in the best interest of the Iranian people," Bush said.

"The choice is up to the Iranian regime," he added. (Writing by Matt Spetalnick, reporting by Jeremy Pelofsky, editing by Vicki Allen

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