MOUNT VERNON, Virginia (Reuters) - The U.S. and French presidents forged a common front against Iran’s nuclear ambitions on Wednesday, signaling a further warming of once-chilly relations between Washington and Paris.

In a sign that diplomatic ties have advanced beyond the era of “freedom fries,” President George W. Bush and French President Nicolas Sarkozy agreed to keep the pressure on Tehran, which has defied demands to halt uranium enrichment.

“The idea of Iran having a nuclear weapon is dangerous,” Bush said standing shoulder to shoulder with Sarkozy at a news conference in Mount Vernon, the historic estate of George Washington, the first U.S. president.

Sarkozy, who has won U.S. praise for taking a stronger stand against Iran than his predecessor, Jacques Chirac, agreed that a nuclear-armed Iran would be “unacceptable” and said there was “a need to toughen the sanctions” against Tehran.

Bush has been ratcheting up his rhetoric toward Tehran and last month raised the specter of World War Three if it were allowed to acquire a nuclear weapon.

Bush and Sarkozy expressed hope that the standoff can be resolved peacefully. Tehran insists it wants nuclear technology strictly for civilian purposes, which Sarkozy acknowledged was its right.

Underscoring how times have changed, Sarkozy trumpeted strengthened ties with Bush in a speech earlier to a joint session of the U.S. Congress that was filled with effusive praise for American values.

“America can count on France in its battle on terror,” Sarkozy said, pledging that his country would stand by the United States in the fight against nuclear proliferation in Iran and terrorism in Afghanistan.

Bush, notably frosty toward Chirac, has been just as eager for a fresh start in relations badly strained by the Iraq war, and called Sarkozy a “a partner in peace.”

VISIT EVOKES LONG ALLIANCE

Sarkozy, 52, France’s most pro-American president in decades, was welcomed on his first official visit to Washington with a warmth normally reserved for the British, whose “special relationship” has made them the closest U.S. ally.

Bush gave Sarkozy a guided tour of the hillside mansion overlooking the Potomac River just south of Washington, D.C. Mount Vernon was chosen to evoke the long alliance between the two countries stretching back more than 200 years to the American War of Independence.

Sarkozy has made no secret of his affinity for American culture, and White House officials say he and Bush used earlier meetings, including an informal get-together at the Bush family compound in Maine in August, to forge close personal ties.

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But mindful of Bush’s deep unpopularity in France, Sarkozy, a fellow conservative, must also show he will not flinch at disagreeing with him. Sarkozy was mocked as “Bush’s poodle” by Socialists before coming into office in May.

Signaling the countries still have their differences, Sarkozy warned lawmakers that by standing by as the dollar falls the United States risks an “economic war” in trade.

Still, it was a far cry from Bush’s chilly relations with Chirac, who rankled the U.S. administration with his outspoken opposition to the U.S.-led invasion of Iraq in 2003.

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That prompted some indignant Americans to make a show of giving french fries the less Gallic name of “freedom fries” and to boycott products such as French cheese.

Taking a swipe at Chirac over Iraq, Bush told Sarkozy, “We had a difference of opinion with your great country over whether or not I should have used military force to enforce U.N. demands.”

Sarkozy sought to avoid any divisiveness and said only that France wanted a united, democratic and peaceful Iraq.

Preferring to dwell on what the United States and France have in common, Sarkozy told lawmakers earlier: “It is together that we must fight to defend and promote the values and ideals of freedom and democracy.”

Sarkozy also said he wanted France to “resume its full role” in NATO, and assured Bush that France would not pull its forces out of Afghanistan.