WASHINGTON (Reuters) - Donald Trump is receiving foreign policy advice from a former U.S. military intelligence chief who wants the United States to work more closely with Russia to resolve global security issues, according to three sources.

Then Defense Intelligence Agency director U.S. Army Lt. General Michael Flynn testifies before the House Intelligence Committee on "Worldwide Threats" in Washington February 4, 2014. REUTERS/Gary Cameron

The sources, former foreign policy officials in past administrations, said retired Lieutenant General Michael Flynn, who was chief of the Defense Intelligence Agency under President Barack Obama from 2012-2014, has been informally advising Trump.

Trump, who is leading the Republican race to be the party’s presidential candidate in November’s election, said earlier this month that he would soon release a list of his foreign policy advisers, but has yet to do so. The Trump campaign did not respond to a request for comment about Flynn.

Flynn declined to comment when asked by Reuters whether he is advising Trump. Asked to describe his views about ties with Russia, he referred Reuters to his public statements.

The question of who has been advising Trump on national security issues has become more pertinent as prospects that the New York real estate mogul will secure the Republican nomination, possibly within weeks, have increased.

Trump won the surprise endorsement of New Jersey Governor Chris Christie on Friday, the most prominent mainstream Republican to come on board.

Former New York Mayor Rudy Giuliani, who won popularity for his handling of the Sept. 11, 2001, attacks, has also been in regular contact with Trump, said a former top aide to Giuliani.

A close associate of Flynn said that Trump was not the only presidential hopeful who had consulted the former DIA chief. “He responds to one and all but is not working for any one,” the associate said.

Trump has struck a notably different stance on Russia from his main rivals for the nomination, calling President Vladimir Putin “highly respected” and advocating a warming of now icy bilateral ties.

Other Republican candidates have frequently taken to bashing Putin and have cited his military interventions in Ukraine and Syria as evidence that President Barack Obama has been weak in standing up to the Russian leader.

Trump has vowed to destroy Islamic State and to undertake an aggressive rebuilding of the U.S. military, but has signaled more flexibility than his rivals on some issues - for example, by not vowing to tear up the Obama administration’s nuclear deal with Iran.

SAT WITH PUTIN

Flynn resigned from his position as the head of the Pentagon’s main intelligence agency a year before his term was officially due to end.

Flynn raised eyebrows among some U.S. foreign policy veterans when he was pictured sitting at the head table with Putin at a banquet in Moscow late last year celebrating Russia Today, an international broadcasting network funded by the Russian government.

His son Michael G. Flynn, who acts as his chief of staff, declined comment on the banquet and on the reasons for his father’s departure from the Pentagon.

Flynn told Russia Today in an interview published on Dec. 10 that the United States and Russia should work together to resolve the Syrian civil war and defeat Islamic State.

The Obama administration has protested Russia’s military intervention on behalf of Syrian President Bashir al-Assad, accusing Moscow of hitting opposition forces rather than ISIS.

“Right now we have essentially the U.S. strategy and we have a Russian strategy in the region that does not appear to be in line with each other. And I think we have to step back and try to figure out how do we align those,” Flynn told Russia Today.

Flynn was also quoted this month as telling German magazine Der Spiegel that the Iraq war launched in 2003 by then-President George W. Bush was a mistake that gave rise to Islamic State.

Trump has often strongly condemned the Iraq invasion.

A former U.S. intelligence official who worked with Flynn said the retired general believes in a more aggressive approach to U.S. interests around the world.

“He’s a sharp guy, he understands foreign policy and national security and really understands intelligence,” said the official. “His positions and opinions are not always in line with popular thinking.”

Giuliani’s office did not respond to a request for comment on his relationship with Trump.

Randy Mastro, a New York lawyer who was a deputy mayor in Giuliani’s New York City administration, said Giuliani has close ties to Trump. “I know that Rudy and Donald Trump have a long-standing relationship and personal friendship that goes back many years, and they do speak to each other on a regular basis,” said Mastro.