Outgoing President Barack Obama condemned Donald Trump as unsuitable to be commander-in-chief Thursday, after the Republican nominee blasted US military brass and praised Vladimir Putin.

"I don't think the guy's qualified to be president of the United States and every time he speaks, that opinion is confirmed," Obama said in unusually caustic language while overseas.

"The most important thing for the public and the press is to just listen to what he says and follow up and ask questions about what appear to be either contradictory or uninformed, or outright wacky, ideas."

Obama holds a news conference at the conclusion of his participation in the ASEAN summit | Source: Reuters

Obama is in Laos for a summit with South East Asia leaders and his final trip to east Asia.

"I can tell you from the interactions I have had over the last eight or nine days with foreign leaders that this is serious business," Obama said.

"You actually have to know what you are talking about and you actually have to have done your homework. When you speak, it should actually reflect thought out policy you can implement."

Obama said he believed his foreign policy legacy would be one of success, particularly his so-called "rebalance" to Asia. He said Asian leaders would be puzzled by Trump's remarks, and Americans would know who to choose as president.

Trump has sparked multiple controversies during his campaign so far | Source: Reuters

"I have confidence that if, in fact, people just listen to what he had to say, look at his track record or lack thereof, that they'll make a good decision," he said.

Trump on Wednesday raised eyebrows by saying that Russia's president was "far more" of a leader than Obama. He also drew fire for criticising the military.

"The generals have been reduced to rubble," Trump said, before noting he had "faith in certain of the commanders".

The bombastic mogul will face Hillary Clinton in November's election, which the Democratic former first lady is tipped to win.