International

US President Barack Obama criticised what he called Donald Trump's adulation of Vladimir Putin, saying the billionaire considers the Russian president a "role model," in a broad attack on the Republican nominee's fitness for the presidency.

"He loves this guy," Obama said at a campaign rally for Democrat Hillary Clinton in Philadelphia. "Think about what's happening to the Republican party. Used to be opposed to Russia and authoritarianism, and fighting for freedom and fighting for democracy."

Obama hit the campaign trail on Tuesday for Clinton, who is recovering from pneumonia at her New York home, and delivered a barn-burning speech in Philadelphia before heading to a fundraiser later in New York City.

He did not mention Clinton's health or her remarks at a fundraiser on Friday that she had to walk back in which she called "half" of Trump's supporters a "basket of deplorables" given to racism and bigotry.

Instead, the president lauded Clinton and levelled repeated attacks on Trump, who he said "isn't fit in any way, shape or form to represent this country abroad and be its commander in chief."

Trump, speaking at a campaign rally in Des Moines, Iowa said little about Obama other than to repeat a promise to repeal the Affordable Care Act and criticise the president for declining to describe the US as in conflict with "radical Islamic terrorism."

Trump continued to try to capitalise on Clinton's "basket of deplorables" remark, accusing her of demeaning "hard-working American patriots."

Obama's criticism of Trump's relationship with Putin was poignant because the US government has alleged that Russian hackers broke into computer systems at Democratic campaign committees and the material was released just before the party convention where Clinton was formally nominated.

Clinton's campaign has charged that Putin's government may be trying to interfere with the US election, a claim Putin denied in an interview with Bloomberg News.

Trump has praised Putin for being a "strong leader," Obama said, after the Russian president "invades smaller countries," jails his political opponents, controls his country's media and has driven its economy into recession.

"I have to do business with Russia," Obama said. "That's part of foreign policy. But I don't go around saying that's my role model."

Obama's own relationship with Putin has been frosty. The US imposed sanctions on Russia after it annexed Crimea from Ukraine in 2014, and Obama has criticised Putin's military intervention in both Ukraine and in Syria, where Putin has sought to bolster the government of his ally Bashar al-Assad by attacking US-backed rebel groups.

The US wants Assad removed from power. In addition, US officials suspect Russian government hackers were behind intrusions into federal computer systems. Russian aircraft have harassed US ships in the Baltic Sea while Putin has criticised NATO's expansion in eastern Europe.

Still, the US and Russia were able to negotiate a fragile cease fire in Syria last week, and Obama and Putin have each said they would like to be partners in fighting terrorism.

Obama also mocked Trump's self-portrayal as a champion of working people -- "he spent most of his life trying to stay as far away from working people as he could," the president said -- and attacked the Republican for refusing to release his tax returns and for the activities of his charity, the Donald J. Trump Foundation.

"You've got one candidate in this race who's released decades worth of her tax returns," Obama said. "The other candidate is the first in decades who refuses to release any at all."

He compared Trump's foundation unfavourably with the Clinton Foundation, which he said "has saved countless lives around the world" through its work.

"The other candidate's foundation took money other people gave to his charity and then bought a six-foot-tall painting of himself," Obama said, referring to a Washington Post report on the Trump Foundation's activities. "He had the taste not to go for the ten-foot version."