US President George W Bush has voiced "deep concern" about European plans to lift an arms embargo on China - putting him at odds with the French president.
China's military build-up has alarmed the Pentagon
Jacques Chirac said it was "no longer justified" but Europe and the US should agree on conditions for lifting it.
Earlier, Mr Bush warned that arms transfers to Beijing would "change the balance" of China-Taiwan relations.
He was speaking after a Nato summit in Brussels that stressed alliance unity.
"There is deep concern in our country that a transfer of weapons would be a transfer of technology to China," Mr Bush said.
He voiced a willingness to consider safeguard clauses, but he hinted that the US Congress could still take action against any move by the Europeans.
The EU imposed a ban on exports of military equipment to China in 1989, following the crackdown on democracy activists in Tiananmen Square.
The BBC's business reporter Mark Gregory says the Pentagon is worried that at some point it might be called upon to defend Taiwan from Chinese attack.
Agreement on Iraq
In a positive signal for the White House, Mr Chirac - a staunch opponent of the US-led invasion of Iraq - said France would join the Nato mission to train Iraqi forces.
Monday: Talks with Belgian leaders and a speech on transatlantic relations. Dinner with French President Jacques Chirac
Tuesday: Breakfast with UK Prime Minister Tony Blair. Meets Ukrainian and Italian leaders at Nato HQ, then meets EU leaders
Wednesday: Leaves Brussels for Germany. News conference with Chancellor Gerhard Schroeder in Mainz, then meets US troops in Wiesbaden
Thursday: Gives speech in Slovak capital, Bratislava, meets Russian President Vladimir Putin
"Europe and the United States are true partners," he told the Nato summit, urging more dialogue.
Mr Bush said Nato was the "cornerstone" of the transatlantic relationship.
All 26 Nato member countries are now helping the Iraqi government, Nato officials announced.
Mr Bush also held talks with European Commission President Jose Manuel Barroso and Luxembourg Prime Minister Jean-Claude Juncker, whose nation holds the rotating EU presidency.
At a joint news conference, Mr Juncker announced plans for the EU and US to co-host an international conference on Iraq - "should the Iraqi government invite us".
The intention, he said, was to encourage and co-ordinate international support for Iraq.