Efforts by U.S. intelligence officials to track American and European-born fighters who travel to the Middle East to join Islamic extremist groups like ISIS have been complicated by different approaches to sharing information and homeland security from their European counterparts, according to a published report.

The Wall Street Journal reports that U.S. officials are struggling to ascertain the movements of suspected extremists once they enter certain European countries. The gaps are occurring despite the fact that the U.S. and several European security services have developed close intelligence links, with intelligence from both sides of the Atlantic buttressing terror watch lists kept by U.S. officials, such as the no-fly list.

According to the Journal, a particular cause for concern among U.S. intelligence officials is a series of anti-terror proposals made last week by British Prime Minister David Cameron, most notably to revoke the passports of British nationals who have traveled to fight for ISIS. The British proposal reportedly has been greeted warily by U.S. counter-terrorism officials, who say that any move to confiscate passports could prevent people who have traveled to Syria and Iraq from speaking to authorities and providing intelligence about what is happening there.

Apparently buttressing the U.S. officials' concerns, a report in The Times of London last week suggested that up to 30 British-born ISIS fighters have been disgusted by the militants' brutal tactics and wish to return home, but are fearful of doing so due to the punitive measures advocated by Cameron.

Meanwhile, President Obama is scheduled to meet with congressional leaders Tuesday afternoon to discuss his plan to combat the ISIS threat. Few details of Obama's plan have been revealed ahead of a scheduled Wednesday address to the nation, though the New York Times reported Monday that the White House was in the process of planning a three-phase campaign that some Pentagon officials believe would take at least three years to fully execute.

The U.S. has already launched close to 150 airstrikes against ISIS in Iraq, and The Times reported that the final phase of the campaign would call for the extension of airstrikes into Syria, where ISIS has its home base.

The Obama administration is also bringing pressure on allies to swing firmly behind action against ISIS. Secretary of State John Kerry is scheduled to travel to to Saudi Arabia and Jordan to meet with Mideast leaders and gauge their level of commitment to a growing worldwide coalition. The Associated Press reported that Kerry pressed a core group of 10 countries  to form a loose coalition to go after last week's NATO summit. Along with the United States, the coalition comprises the United Kingdom, France, Australia, Germany, Canada, Turkey, Italy, Poland and Denmark.

As he weighs his next move, Obama was soliciting advice Monday from prominent foreign policy experts from across the political spectrum over dinner at the White House. Among the guests invited to join Obama and Vice President Joe Biden were former national security advisers from the Obama, George W. Bush, Clinton and Carter administrations, as well as Council on Foreign Relations President Richard Haass and former Acting CIA Director Michael Morrell.

In a call Monday evening, Obama congratulated new Iraqi Prime Minister Haider al-Abadi for the approval of a new government. The White House said al-Abadi "expressed his commitment to work with all communities in Iraq as well as regional and international partners to strengthen Iraq's capabilities" to fight the Islamic State militants.

Obama also spoke with Australian Prime Minister Tony Abbott on the need to keep addressing the ongoing threat from the Islamic State and to thank Australia for its contributions to humanitarian air drops in northern Iraq, the White House said.

The Associated Press contributed to this report. 

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