WASHINGTON (Reuters) - Leading Republican U.S. presidential candidate Donald Trump on Tuesday said he supported Russian efforts to fight Islamic State militants in the Middle East, including Syria.

Asked whether he backed those like Russia who supported Syrian President Bashar al Assad or those who see him as the source of Syria's current crisis, Trump told NBC's "Today" program: "I side with the group that says 'if Russia wants to go and fight ISIS, you should let them', as opposed to saying 'we're jealous, we don't want you to do that'."

Trump, who is leading public opinion polls among those seeking the Republican Party's bid to win the White House in the 2016 election, said the United States should support those who want to destroy the militant group that has taken over swaths of Syria as well as neighboring Iraq.

Republicans have criticized Democratic U.S. President Barack Obama's foreign policy in Syria, which has been mired in civil war for four years and has seen an influx of Islamic State militants.

Asked about whether Assad was the source of the country's ills, Trump said it was not clear and questioned who would replace him if he were ousted.

"The people that want to come in and replace Assad, nobody knows who they are and they could end up being worse," he said. "We're constantly going out and siding with people and they turn out to be worse than the people who were there before."

(This version of the story was refiled to fix punctuation in quote in paragraph two)

(Reporting by Washington Newsroom)