The US ambassador to Iraq has spoken of differences between the US and Britain over future strategy for Iraq.
Zalmay Khalilzad said the US and UK were holding talks over Iraq
The government is planning a partial withdrawal of British forces from Basra this year.
But US ambassador Zalmay Khalilzad told the BBC the US would prefer UK forces to remain at their current levels in southern Iraq.
However, Tony Blair's spokesman has denied any differences with the US over British troop levels in Iraq.
The BBC's Andrew North said it appears to be the two allies' "first sign of disagreement" over Iraq since 2003.
The BBC's Baghdad correspondent said for the first time since the invasion four years ago, Britain and the US appear to be taking a "different approach" over Iraq, with the Americans increasing their force levels while Britain planned a partial withdrawal.
Mr Khalilzad told our correspondent: "We would like the British to co-ordinate and for us to have a joint plan. We're talking about this. It's clear what our preference would be - the longer we stay together here, the better."
He said talks between Washington and London over the issue were continuing and he had "no doubt" that a "mutually acceptable" agreement could be reached.
Mr Khalilzad added: "The Brits are our great allies, the prime minister is a great leader and we have worked together very well here. We worked together at all levels very well here, so I have no complaints."
Mr Blair's official spokesman said it was "hardly surprising" that discussions were still going on while British troops continued their operations in Basra.
"The prime minister has said that no final decision has been made about our security profile in Basra," he said.
"That will be decided after our assessment of Operation Sinbad. Therefore we will continue to be in discussions with our commanders on the ground and our coalition partners about what are the right levels of our security profile in Basra.
"The prime minister, once he has made his decision, will then make a statement in the House."
State Department spokesman Sean McCormack said the US and UK were working side by side and there was "no daylight between us".
He described the relationship with the UK on Iraq and other foreign policy matters as "terrific".
The Ministry of Defence also said there was "no tension between US and UK objectives and activities in Iraq".
A MoD spokeswoman said: "Our plans are consistent with the coalition's long-term strategy and the Iraqi government's desire for increased Iraqi security responsibility.
"This is the strategy of transition, which we are working towards, as are the Americans. We always said this was a process and not an event and will happen at a different pace across Iraq."
The Shadow Foreign Secretary, William Hague, said the ambassador's comments indicated there were questions over the co-ordination of policy and no evidence the UK view had been "fully put".
"The implications of that, if it is not co-ordinated, are very serious for our troops and for the success of operations in Iraq," he said.
Earlier, Mr Blair dismissed a call by Liberal Democrat leader Sir Menzies Campbell for UK troops to leave Iraq by October.
During prime minister's questions, Mr Blair said setting an "arbitrary timetable" would send a "disastrous signal" to Iraq.
"It is a policy which, whatever its superficial attractions may be, is actually deeply irresponsible," he added.
Sir Menzies is the only one of the three main UK party leaders to suggest a withdrawal date.