A foreign policy adviser to Republican presidential candidate Donald Trump believes the United States should work more closely with Russia on global issues such as defeating the Islamic State group.

Retired Lieutenant General Michael Flynn, who was President Barack Obama's Defense Intelligence Agency chief from 2012 to 2014, also has said that the Iraq war was a mistake that helped create IS, among a number of statements that have defied Republican orthodoxy.

Flynn has openly acknowledged advising frontrunner Trump and other candidates. Like Flynn, Trump has strongly condemned the U.S. invasion of Iraq in 2003.

And like Flynn, Trump has taken an unorthodox stance on Russia, advocating a warming of relations and calling Vladimir Putin a "highly respected" president who he would get along with.

Other Republicans frequently bash Putin and his military interventions in Ukraine and Syria.

Flynn raised eyebrows last year when he sat with Putin at a banquet in Moscow celebrating Russia Today, a Russian state broadcasting network.

Flynn told Russia Today in a December interview that the United States and Russia should work together to resolve the Syrian civil war and defeat IS, rather than work at cross purposes.

Based on reporting by Reuters, CNN, and U.S. News & World Report