NAJAF, Iraq (AFP) -- Iraq said on Tuesday it will reject any security pact with the United States unless it sets a date for the pullout of US-led foreign troops, a proposal turned down by US President George W. Bush.

The controversial demand from Baghdad's Shiite-led government underlines Iraq's new hardened stand in complex negotiations aimed at striking a security deal with Washington.

"We will not accept any memorandum of understanding if it does not give a specific date for a complete withdrawal of foreign troops," national security advisor Muwaffaq al-Rubaie told reporters in the holy city of Najaf.

Rubaie said it was proving "very difficult" to set such a date.

"Tthe Iraqi government has spoken about its date, while the foreign party has spoken about its date," he said. "Until now we have not arrived at an agreement on this issue."

Baghdad and Washington are negotiating a deal that would see the presence of US-led forces beyond 2008 when the UN mandate which provides the legal basis for a foreign troop presence in Iraq expires.

The security pact, also known as Status of Forces Agreement (SOFA), has to be signed by July 31 according to a previous agreement between Bush and Maliki.

On Monday, Iraqi Prime Minister Nuri al-Maliki told Arab ambassadors in Abu Dhabi that he was seeking a timetable for the withdrawal of foreign troops as part of the security agreement.

"The direction we are taking is to have a memorandum of understanding either for the departure of the forces or to have a timetable for their withdrawal," Maliki was quoted as saying.

The US State Department refused to comment on Rubaie's remarks.

"During this entire process, we have said a couple of things consistently -- number one, this is going to be a negotiation, discussion between sovereign countries; number two, we at least are not going to be talking about any components of this during the process," said State Department spokesman Gonzalo Gallegos.

On Monday, the White House reacted to Maliki's comments by saying it was not negotiating a "hard date" for a US withdrawal from Iraq but it did not rule out discussions on "time-frames" with Baghdad.

White House spokesman Scott Stanzel said the talks were aimed at reaching agreement on a framework for future US-Iraqi relations and on the arrangements to govern the US military presence.

"It is important to understand that these are not talks on a hard date for a withdrawal," he said. "When you make an agreement," he added, however, "that doesn't mean that there won't be some understanding of time-frames."

The negotiations have been more difficult than expected, and the prospect of an agreement in the final months of the Bush administration has aroused controversy in political circles in both Iraq and the United States.

Shiite and Sunni politicians have raised objections, and Democrats in the US Congress have expressed fears that any agreement would tie the hands of the next president.

The sticking points have been the duration of a defence pact, how many bases Washington should retain, what powers the US military should continue to hold to detain Iraqis, and what immunity US troops should have.

The negotiations come amid a marked improvement in the security situation in Iraq that has allowed a drawdown of US forces, which now number 146,000, down from over 160,000.

On Tuesday, Republican White House candidate John McCain warned that security conditions must dictate troop withdrawals.

But his Democratic foe Barack Obama, an early opponent of the war who wants to pull out most combat troops within 16 months, said it was encouraging that Iraq now wanted to work out a timetable for withdrawal.

"We will withdraw, but... the victory we have achieved so far is fragile and (the redeployment) has to be dictated by events and on the ground," McCain said in an interview with MSNBC.

Obama said Maliki's remarks were in line with his own policy.

"I think it's encouraging... that the prime minister himself now acknowledges that in cooperation with Iraq, it's time for American forces to start sending out a timeframe for the withdrawal."