Baghdad's ties with Washington will likely stay close even after US troops depart, but neighbouring countries including Iran will probably make plays for increased influence here, experts say.

All US troops except for a small number of trainers are to depart by the end of December, but Iraq will still host the largest American embassy in the world, with the US mission including up to 16,000 people nationwide.

Ali al-Saffar, an Iraq analyst with the Economist Intelligence Unit in London, said he expected the US-Iraq relationship will be "quite close, much like the relationship between the US and pretty much the rest of the region."

"The Iraqis understand that they need to have US companies in Iraq and that there's going to probably be quite an important trade relationship between the two countries well into the future," Saffar said.

"That's primarily, in the short-term anyway, going to be based around, probably, two things" -- oil services contracts and arms deals, he said.

The Office of Security Cooperation-Iraq, under the authority of Washington's embassy, includes 157 US military personnel and up to 763 civilian contractors who will train Iraqi forces on American military equipment Baghdad buys.

Iraq is also seeking to dramatically ramp up its oil output around fourfold in the coming years. Baghdad is almost entirely dependent on crude for government income.

"I think that obviously the US troop withdrawal will mean that there's less influence, less US influence," Saffar said.

He said that while he did not think that will leave major security or political gaps here, "there will be continued attempts to try to influence the direction Iraq takes, clearly from countries like Iran and Saudi Arabia."

"Iraq is a big prize in the region," he said.

"But Iraq hasn't always been a place that can yield to external kind of pressures or influence. ... And so I'm not sure it can become a proxy of any country in the region."

However, if there is a return to the bloody sectarian conflict that once gripped the country, "that will give a lot of space for regional powers ... mainly Saudi Arabia and Iran, to get involved," he said.

US President Barack Obama on Monday vowed an "enduring" partnership with a "self-reliant" and sovereign Iraq, and warned other countries not to interfere in Iraq's affairs.

"Just as Iraq has pledged not to interfere in other nations, other nations must not interfere in Iraq," Obama said after meeting Iraqi Prime Minister Nuri al-Maliki at the White House, in a clear warning to Iran not to try to meddle in its neighbour's affairs.

US-Iraq ties will remain close "for a while at least," said Joost Hiltermann, an Iraq specialist and Middle East and north Africa deputy programme director for the Brussels-based International Crisis Group.

He noted the substantial military purchases Iraq has made from the US, and also that "the US will also be involved in police training for a while, and there are other dimensions of the bilateral relationship which may or may not work out."

"The main thing is that Iraq will now approach the US as a partner, not as an occupier, and will insist on terms that are advantageous to itself. Because the US needs Iraq against Iran, it will have to look favourably at those terms," he said.

"Iran and Turkey are the main regional contenders," said Hiltermann.

"Turkey is investing heavily throughout the country" while "Iran is pursuing a security relationship, and is encouraging cross-border trade and tourism," he said, adding that "each will be successful in its own way, and there is room for both."

According to John Drake, an analyst with Britain-based private security firm AKE, the future of the US-Iraq relationship could depend on how Baghdad cultivates its regional ties.

"Iraq's relationships with its immediate neighbours are also undergoing change, and this in turn could have an impact on the relationship with the US if Iraq becomes more closely aligned with states opposed to US influence in the region, such as Iran," he said.

The post-withdrawal "vacuum in Iraq will likely be filled by many players, ranging (from) private companies from Europe and Asia with a variety of different interests and commercial agendas, to foreign states," Drake said.

"Neighbouring countries will be intent on influencing developments in the country to ensure that Iraq does not evolve into a threat, and indeed that it remains an opportunity for their strategic interests."

But "the fear is that Iraq's neighbours will be forced into a position of competition against one another in the country, sponsoring political organisations that support their influence in the country, fostering conditions conducive towards a proxy conflict" in Iraq, he said.