The US and two leading Nato members have clashed over the alliance's contribution to Iraqi security.
Nato ministers met in Brussels to welcome seven new members
US Secretary of State Colin Powell said Nato should consider an expanded role in Iraq.
But the German and French foreign ministers - who met Mr Powell and other Nato counterparts in Brussels on Friday - immediately rejected the call.
The disagreement surfaced as Nato welcomed seven countries from Central and Eastern Europe as new members.
Russia, which is wary of the expansion, has stressed the importance of co-operation against new threats.
"The United States believes the alliance should consider a new collective role after the restoration of Iraqi sovereignty," Mr Powell said.
But Germany's Foreign Minister, Joschka Fischer, said Nato was already "overstretched" with its missions in the Balkans, Afghanistan and its efforts to fight terrorism.
His new French counterpart, Michel Barnier, said the issue was "not topical".
"We think Nato is simply not the right place where
a decision should be prepared or taken concerning the situation in Iraq after 1 July," he said, referring to the date of the planned handover of sovereignty to a caretaker government in Iraq.
Mr Powell had suggested Nato could take over a sector in Iraq or train Iraqi security forces.
But many in Nato have been insisting on a new United Nations Security Council resolution before deploying the alliance's troops in Iraq.
The US-led invasion of Iraq split the alliance last year.
Since then, Nato has limited itself to providing logistical
support for a Polish-led division in south-central Iraq.
Meanwhile, Russia has been commenting on the expansion of Nato.
President Vladimir Putin conceded "the encroachment of Nato military infrastructure to our borders" would affect defence planning.
But he insisted Russia had "no concerns about the expansion of Nato in terms of the security of the Russian Federation" - though he warned it would not make "today's threats" disappear.
Mr Putin's Foreign Secretary, Sergei Lavrov, stressed that Russia must be included in "co-operative structures".
He said such thinking was not based on paranoia about Nato's eastward advance, but that nations had "to find new ways of dealing with threats rather than set up new fences to divide us".
Nato foreign ministers issued a statement after their meeting in Brussels, outlining new measures to increase co-operation in the fight against terrorism.
Their pledges included:
- More sharing of intelligence
- reinforcing security for this summer's Athens Olympics and European football championships in Portugal
- Co-ordinating Nato assets to deal with nuclear, chemical and biological attacks
- Enhanced support if a Nato member suffered a terror attack.
The ministers also approved plans to expand the alliance's peacekeeping mission in Afghanistan beyond Kabul, and into five northern towns.
On Friday morning, a ceremony was held at Nato headquarters to welcome seven former communist countries as members of the 55-year-old alliance.
The flags of Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia were raised for the first time at the Nato headquarters in Brussels.
The three Baltic republics, Estonia, Latvia and Lithuania, were once part of the Soviet Union - the very power whose military might and perceived threat led to Nato being created.