WASHINGTON - The United States on Thursday hailed the Iraqi parliament's approval of a landmark accord for US troops to leave the country in three years, but a referendum on the deal next year could complicate withdrawal plans for the next US president.

President George W. Bush said the parliamentary vote was a watershed for Iraq and its relations with the United States.

The accord that calls for US forces to withdraw by the end of 2011 would "formalize a strong and equal partnership," US Ambassador to Iraq Ryan Crocker and General Ray Odierno, the top commander of US troops in Iraq, said in a joint statement.

But as part of political bargaining leading up to the vote, the Baghdad government agreed to demands by Sunni parties to hold a referendum on the accord no later than July 30.

The move injects a fresh element of uncertainty on the future of US troops in Iraq just as president-elect Barack Obama prepares to take office on January 20.

"The referendum would create a potentially dangerous political wild card, especially since it would occur around the time of next year's national elections," said the National Security Network, a Washington think tank allied with the Democratic party.

"The possible consequences of such a referendum are unclear but may well complicate the prospects for a smooth withdrawal," it said.

Should the Iraqi government decide to cancel the accord after the referendum it would have to give Washington one year's notice, meaning that troops would be allowed to remain in the country only until the summer of 2010.

The international agreement will be binding on Obama when he becomes president next year, but he could also unilaterally cancel the pact with a year's notice or withdraw all US troops at any time.

The deal -- comprising two separate agreements governing the US military presence in Iraq after December 31 -- was approved by a large majority in the parliament after 11 months of difficult talks.

Iraq's top negotiator and serving national security advisor Muwafaq al-Rubaie insisted that Washington would have to accept the decision to hold a referendum.

"It is an Iraqi issue and the Americans have to understand our requirements," al-Rubaie said.

It was too early to forecast the outcome of a popular vote in Iraq's volatile political environment, said Michael O'Hanlon, a senior fellow at the Brookings Institution who focuses on the Iraq conflict.

"Predictions are already hard in Iraq and eight months out in the future is even more difficult. It is a wild card and that is about as much as we can say right now," O'Hanlon said.

Obama's administration would have to prepare contingency plans for an earlier withdrawal if the agreement was rejected by Iraqi voters, he said.

"You have to have a backup plan to figure out what to do if they vote it down. At least you have eight more months to cope with that wild card," O'Hanlon said.

Apart from the referendum, there were also legal questions about the precise meaning of the agreement, with the Bush administration releasing the official English translation of the accord only after the parliament approved it.

"We thought it was an appropriate time to release the text after it had been agreed upon by the Iraqi parliament," said Carlton Carroll, assistant press secretary at the White House.

He indicated that releasing it earlier might have disrupted sensitive talks on the terms of the agreement.

A report from McClatchy newspapers, citing unnamed US officials, said the administration had withheld the English translation in a bid to avoid a public feud with Iraqi leaders and that some provisions remained in dispute.

"There are a number of areas in here where they have agreement on the same wording but different understandings about what the words mean," said a US official speaking on condition of anonymity.

The agreement on the future of US troops was made possible in part by security improvements over the past year.

The Bush presidency has been indelibly marked by the Iraq war, from the invasion spurred by lies that Saddam Hussein was harboring weapons of mass destruction to the abuses and torture by US troops of Iraqis in the Abu Ghraib jail and other jails.

Some 4,200 US soldiers have been killed in the country in a war which has also cost the US hundreds of billions of dollars.

Figures for Iraqi deaths are unclear, but some estimates put it at around one million.