Donald Trump says Russian President Vladimir Putin is a better leader than US counterpart Barack Obama, as the Republican presidential nominee argued he is the best equipped to reassert America's global leadership.

Mr Putin is "very much of a leader," Mr Trump said in a televised interview, where he and Democratic nominee Hillary Clinton were separately grilled over their national security and military credentials.

Mr Trump has made no secret of his admiration for Mr Putin, who last year praised the US businessman as "very outstanding".

Mr Putin "has very strong control over a country," Mr Trump said.

"It's a very different system, and I don't happen to like the system.

"But certainly in that system he's been a leader, far more than our president has been a leader."

Mr Trump's praise of Mr Putin and his suggestion that the United States and Russia form an alliance to defeat Islamic State (IS) militants could raise eyebrows among foreign policy experts, who feel Moscow is interfering with efforts to stem the Syrian civil war.

"If he says great things about me, I'm going to say great things about him," Mr Trump said of Mr Putin.

Mr Trump called Mr Obama "the founder of ISIS" in stump speeches several weeks ago.

The statement drew broad criticism, prompting him to take a more disciplined approach to campaigning.

He has since picked up ground on Mrs Clinton in national opinion polls.

Mr Trump also pledged to increase US military spending — already at levels far higher than any other nation — and demanded a plan to beat IS if he becomes president.

Sorry, this video has expired Donald Trump pledges military expansion

Reuters/AFP