WASHINGTON (REUTERS) - The United States is intensifying its push to build an international campaign against Islamic fighters in Iraq and Syria, including recruiting partners for potential joint military action, Obama administration officials.

Britain and Australia are potential candidates, US officials said on Wednesday.

Germany said it was in talks with the United States and other international partners about possible military action against the Islamic State fighters, but made clear it would not participate.

"We are working with our partners and asking how they might be able to contribute. There are a range of ways to contribute: humanitarian, military, intelligence, diplomatic," State Department spokesman Jen Psaki told reporters.

It's unclear how many nations will sign up. Some such as trusted allies Britain and France harbour bitter memories of joining the US-led "coalition of the willing" in the 2003 invasion of Iraq that included troops from 38 nations.

The claims of the existence of weapons of mass destruction which spurred the coalition to act were found to be false.

The United States, the officials said, could act alone if necessary against the militants, who have seized a third each of Iraq and Syria, declared open war against the West and want to establish a hub of jihadism in the heart of the Arab world.

Senior White House aides met this week to discuss a strategy for expanding its assault on Islamic State, including the possibility of air strikes on the militants' stronghold in eastern Syria - an escalation that would almost certainly be riskier than the current US campaign in Iraq.

While Iraq's government welcomed the role of US war planes to attack the militants, Syrian President Bashar al-Assad has warned that any strikes conducted without its permission would be considered an act of aggression, potentially plunging any US-led coalition into a broader conflict with Syria.

The British embassy in Washington said it has not received a request from the United States for air strikes. A spokesman for Australian Prime Minister Tony Abbott said humanitarian aid in Iraq could continue but declined to say whether Australia would join US-led military action.

"Our response to any request from the United States, or other close allies and partners, will be based on whether there is an achievable overall humanitarian purpose and a clear and proportionate role for Australia as well as on a careful assessment of the risks," Abbott's spokesman said.

US officials hope the relative success of humanitarian aid and recent strikes on militant weapons in Iraq will diminish allied fears over supporting new military action.

"There's been a proof of concept in Iraq that with a limited campaign you can do (things) against these guys," one official said on condition of anonymity. "What we did already (has yielded) 25 to 30 nations offering to help," the official added, referring to widespread international humanitarian aid.

RELUCTANT ALLIES?

Among America's possibly reluctant allies is France, which was left on a limb when US President Barack Obama backed down from the threat of strikes on Syria following a major chemical attack a year ago.

"It was embarrassing for us," a senior French diplomat said. "After what happened last year, now when the Americans decide to do something we will need some very strong guarantees before committing to anything."

More broadly, American officials appear to be accelerating efforts to build a wide coalition of countries that can sap Islamic State's strength in both Iraq and Syria.

Those talks include a large group of countries, many of whom are eager to weaken Islamic State but are unwilling to take part in military action.

That list includes Turkey, Saudi Arabia, the United Arab Emirates, Qatar, Jordan, Britain, France, Australia and Germany, two officials said on condition of anonymity.

The beheading of American photojournalist James Foley and news of a growing number of US and European passport holders now fighting alongside the Islamic State add urgency to Washington's concerns over the rise of the al-Qaeda offshoot.

White House spokesman Josh Earnest said Obama would convene a United Nations Security Council meeting in September on the threat posed by radicalised residents of Western nations returning from fighting in Syria.

French officials are planning another conference next month that would bring together Western and Arab nations in an effort to better coordinate efforts against Islamic State.

"The objective is to get a comprehensive view of how to deal with (Islamic State) both in Iraq and Syria. See what each country is willing to do and discuss what to do about Syria and Assad," one diplomat said.