(Adds U.S. comment, paragraphs 6, 9)

* Demand for changes exasperates Washington

* Iraqi says Washington agrees to hear proposals

* U.S. pressures Iraq to accept pact or state objections

By Mariam Karouny and Peter Graff

BAGHDAD, Oct 22 (Reuters) - Baghdad will demand changes to the wording of a pact allowing U.S. troops to stay in Iraq but will not seek to renegotiate the "backbone" of the agreement, Foreign Minister Hoshiyar Zebari said on Wednesday.

"In my opinion and based on my follow-up for the negotiations, I do not think there will be structural amendments. Maybe it will touch the wording and descriptions, possibly, but the backbone of the pact is what has already been agreed on," Zebari told Reuters in an interview.

Iraq's cabinet decided on Tuesday to demand amendments to the pact, despite having agreed last week to a "final draft" after months of painstaking negotiations with Washington.

The decision to reopen the negotiations has exasperated Washington, which is worried that its troops could have no legal basis to remain in Iraq beyond the end of this year when a U.N. Security Council resolution authorising the force expires.

"There is a great reluctance to engage further in the drafting process," U.S. Defence Secretary Robert Gates said on Tuesday. "This is not just kind of a paper exercise. The consequences of not getting an agreement would be real."

In a sign of the frustration, a State Department official in Washington, who asked not to be named, said one consequence would be that U.S. forces would have to stop providing personal protection to senior Iraqi officials.

So far, Iraqi leaders have been circumspect about what they object to in the draft, which would require U.S. forces to leave Iraq by the end of 2011 and also provides a mechanism for Iraqi courts to try American troops for serious crimes committed off duty.

The government has said it will draw up amendments soon and submit them to U.S. negotiators.

CRUCIAL DAYS

The United States pressed Iraq to accept the pact or state its objections. "We're running out of time," said State Department spokesman Robert Wood. "The door is closing and ... it's time for the Iraqis to step up to the plate and make a decision."

Zebari said the Americans have agreed to hear the proposals though he was unsure whether they would accept them.

"The coming days will be crucial to determine the fate of this pact," he said. "We were told that they are ready to look into the amendments but to what extent they will accept them we do not know. We are not at that stage yet."

If the pact cannot be agreed by the end of the year, Iraqi officials say they could seek an emergency extension of the existing U.N. mandate. Zebari said an extension would let Baghdad negotiate with a new U.S. administration.

"We must begin as an Iraqi government to prepare in case there is no pact. We should look into other alternatives. One of them is to go to U.N. Security Council and ask them to extend the mandate for another year or six months until the new administration comes. We have to be ready for such an option."

But he made clear agreeing the pact was preferable to extending the mandate: "The absence of this pact will expose the country to dire consequences security-wise, economically and internationally," he said. "The time window is shutting slowly."

The pact has exposed fault lines in Iraq's ruling coalition of majority Shi'ite Arabs and minority Kurds.

Kurdish factions, whose members include Zebari and President Jalal Talabani, endorsed it and seem to have been caught off guard by the objections raised by others. Just days ago, Zebari said the draft was final and unlikely to be renegotiated.

But the Shi'ite alliance of Prime Minister Nuri al-Maliki was among the first to demand amendments. Maliki himself has yet to speak about the draft publicly, even though he hand-picked the team that negotiated it.

It may be politically difficult for Shi'ite parties to support the pact. Maliki's backers face an important election challenge early next year from followers of cleric Moqtada al-Sadr, who has generated popular support among Shi'ites by opposing it.

Mainly Shi'ite Iran, which virulently opposes a pact it contends would give its arch-foe a foothold in the region, also has influence among Iraqi Shi'ite politicians. Washington has accused Tehran of pressing Iraqi parliamentarians to block it. (Additional reporting by Arshad Mohammed in Washington, editing by Chris Wilson)

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