By Phil Stewart

BAGHDAD (Reuters) - The U.S. military expects to seek additional troops in Iraq, even beyond the hundreds announced this week, as the campaign against the Islamic State advances, the head of the U.S. military's Central Command told Reuters.

"As we continue on the mission, I think there will be some additional troops that we will ask to bring in," U.S. Army General Joseph Votel said in an interview in Baghdad on Thursday, without disclosing a number.

Votel, who oversees U.S. forces in the Middle East, said the size of possible future increases were still being discussed within military circles. He did not offer details on the timing of any requests to President Barack Obama's administration.

His remarks came just three days after Obama's administration announced a 560 troop increase as part of an effort to facilitate an Iraqi offensive to retake Mosul, Iraq's second biggest city.

Most of those troops will work out of Qayara air base, which Iraqi forces recaptured from Islamic State militants last week.

They plan to use Qayara as a staging ground for an offensive to retake Mosul.

Votel suggested future requests would similarly be tailored to particular stages of the campaign.

"We try to tie our requests to specific objectives we're trying to achieve on the ground," he said.

The recapture of Mosul, Islamic State's de facto Iraqi capital, from which its leader declared a modern-day caliphate in 2014, would be a major boost for the plans by Iraqi Prime Minister Haider al-Abadi and the United States to weaken the militant group.

Abadi has pledged to retake Mosul by the end of the year.

Some U.S. officials caution that retaking the city without a plan to restore security, basic services and governance would be a major mistake and question the ability of Iraq's Shi'ite-government in Baghdad to mend the sectarian divide fueling the conflict.

Votel broadly acknowledged concerns about the non-military aspects of the campaign but said he felt more upbeat after meetings on Wednesday with top Iraqi officials, including Abadi.

"While there is still a lot of work to do – a lot of work to do – I left more encouraged," he said, stressing the importance that U.S.-backed military operations "pay off on the political side."

With the latest troop increase, the United States has an official limit of just over 4,600 troops formally assigned to Iraq, although the actual figure is higher due to temporary assignments.

Obama has opposed recommitting the United States to another large-scale ground war in the Middle East and any deployment of forces to Iraq would likely need to be measured.

Republican leaders this week called on Obama to ask Congress for additional funds to pay for the deployment of more troops to Iraq, as Congress and the White House debate defense spending amid mandatory budget cuts.

NO WITHDRAWAL

As Islamic State militants have lost part of their self-proclaimed caliphate in Iraq and Syria, they increasingly have turned to suicide attacks.

These included a bombing in the Iraqi capital last week that left nearly 300 people dead, the most lethal bombing of its kind since the 2003 U.S.-led invasion.

Votel was speaking before a gunman killed 80 people and wounded scores when he drove a heavy truck at high speed into a crowd watching Bastille Day fireworks in the French Riviera city of Nice. No group has claimed responsibility.

Votel cautioned that even after Islamic State eventually loses Mosul and the Syrian city of al-Raqqa, Americans should not expect a rapid, wholesale withdrawal from the country. "What we don't want to do is declare victory and depart after that. I think we want to see this through," Votel said.

If Islamic State fighters shift to other locations, outside those cities, Votel said it was important to have U.S. military resources in place "to ensure we can achieve that lasting defeat."

"If there's capabilities we don't need, we will remove them. Likewise if there's capabilities we do need that we don't have, we'll ask for them," Votel said, describing an evolving campaign that won't end soon.

(Reporting by Phillip Stewart; Editing by Robert Birsel)