Sixteen countries want to start military planning for the defence of Turkey in the event of a war in Iraq.
But after a meeting of Nato ambassadors on Wednesday, France, Germany and Belgium are still blocking the plan - saying it could undermine efforts to find a diplomatic solution to avert war with Iraq.
However, Nato spokesman Yves Brodeur said the allies were still "involved in intense consultations" and the ambassadors are due to meet again later on Wednesday.
The Bush administration has warned that decision time is rapidly approaching for both Nato and the United Nations regarding Iraq.
On Friday, the UN's chief weapons inspectors, Hans Blix and Mohammed ElBaradei, are to present a crucial report on whether Iraq has been complying with disarmament demands.
Intense talks
The BBC's Oana Lungescu in Brussels says a new compromise plan presented by Nato Secretary General George Robertson involves deploying Patriot anti-missile batteries, Awacs surveillance planes, and chemical-biological protection units in Turkey.
US REQUEST FOR TURKEY
After a night of intense negotiations, she says, the US appears to have dropped two other requests - the replacement of US troops in the Balkans with European soldiers, and increased security at US bases around Europe.
The US ambassador to Nato, Nicholas Burns, said the alliance was "obligated" to provide military support to Turkey.
However, France on Wednesday said it had not changed its position.
"We cannot through a Nato decision today give our unconditional support to armed intervention in Iraq and thereby prejudge decisions which are the responsibility of the Security Council," French foreign ministry spokesman Francois Rivasseau said.
As the diplomatic deadlock continues, so does the US military build-up in the Gulf.
Pentagon officials say there are now at least 130,000 US personnel within striking distance of Iraq, and they are ready to begin operations if necessary.
The Pentagon is still hoping to get the green light from Turkey to station at least one army division there to open a potential northern front, says the BBC's Nick Childs.
That is one reason Pentagon planners have been so angry about the row in Nato over defending Turkey.
In other developments:
In London, UK Prime Minister Tony Blair outlined the "moral" case for war against Saddam Hussein, saying the alternative was a sanctions regime that could result in the deaths of thousands of Iraqis.
Mr Blair told Parliament that keeping the sanctions was also "a choice with bad and devastating consequences for the Iraqi people".