WASHINGTON (REUTERS) - US Republican presidential nominee Donald Trump ratcheted up tensions in his party on Tuesday (Aug 2) by denying support in the re-election bids of two leading figures, House of Representatives Speaker Paul Ryan and Senator John McCain.

Mr Trump told The Washington Post in an interview that he could endorse neither Mr Ryan, the top elected Republican, nor Mr McCain, a senator from Arizona and a former Republican presidential nominee, as they face challenges in their states'primary contests ahead of the Nov 8 general election.

Both Mr Ryan and Mr McCain had criticised Mr Trump's feud with the family of Army Captain Humayun Khan, who died in the line of duty in Iraq in 2004 and was awarded the Bronze Star Medal for bravery after his death.

The discord comes just two weeks after the Republican National Convention in Cleveland that formally nominated Mr Trump for president.

It is the latest rift in a party already frayed by internal dissent over its standard bearer, seen in stark relief at the convention where Mr McCain was among high-level party members who essentially snubbed Mr Trump by choosing not to attend. Mr Mitt Romney, the 2012 Republican nominee, and former presidents George H.W. Bush and George W. Bush also did not attend the convention.

Mr Trump has had a running dispute with the parents of the slain soldier, Khizr and Ghazala Khan, since they took the stage at last week's Democratic convention to cite their son's sacrifice and criticise Mr Trump's proposed ban on Muslims entering the United States.

The uproar has led many Republicans to distance themselves from Mr Trump and voice support for the Khan family.

Mr Trump, mirroring the language Mr Ryan used about supporting the nominee before his eventual endorsement, told the newspaper he was "not quite there yet" on endorsing the latter in next Tuesday's Wisconsin primary, and that he had "never been there" with Mr McCain, who will be on the ballot in primary elections in Arizona later this month.

Mr Trump said Mr Ryan had sought his endorsement, but that as of now he is only "giving it very serious consideration."

Mr Ryan's campaign office quickly responded that "neither Speaker Ryan nor anyone on his team has ever asked for Donald Trump's endorsement."

"And we are confident in a victory next week regardless," campaign spokesman Zack Roday said in a statement.

Mr Ryan is favoured to win against primary challenger Paul Nehlen, whom Mr Trump praised as running "a very good campaign." In a mid-July survey by Harper Polling, Mr Ryan was ahead of Nehlen by nearly 50 points.

Mr Trump, a former reality TV star, has troubled many in the Republican establishment with his off-the-cuff, often insulting style, and controversial policies, including the proposed ban on Muslims and his plan to build a wall along the Mexican border to keep out illegal immigrants.

President Barack Obama on Tuesday blasted Mr Trump as unfit to be president and questioned why any Republican would support the New York businessman, who is seeking his first public office.

"The question I think that they have to ask themselves is, if you are repeatedly having to say in very strong terms that what he has said is unacceptable: Why are you still endorsing him?" Mr Obama, a Democrat, said at a White House news conference with Singapore's prime minister Lee Hsien Loong.

CONGRESSMAN ABANDONS TRUMP

The chorus of complaints against Mr Trump grew on Tuesday.

Representative Richard Hanna of New York became the first Republican in Congress to endorse Democratic nominee Hillary, although several other Republicans in Congress have said they will not support Mr Trump.

Mr Hanna, who is retiring from the House of Representatives rather than seek re-election, said his decision was prompted by Mr Trump's attacks on the Khan family. He called Mr Trump "deeply flawed in endless ways," "unrepentant" and "self-involved."

New Jersey Governor Chris Christie, a Trump ally once viewed by the billionaire as a potential running mate, called criticisms of the Khan family "inappropriate."

Mr Trump's son Eric told CBS News on Tuesday that his father's comments about the Khans have been "blown hugely out of proportion."

The tycoon has fallen behind Mrs Clinton in opinion polls made public since the parties held their nominating conventions last month.

Mr Clinton extended her lead over Mr Trump to 8 percentage points, according to a Reuters/Ipsos poll released on Tuesday, from 6 points on Friday. About 43 per cent of likely voters favour Mr Clinton, 35 per cent favour Mr Trump, and 9 per cent picked "other."

Mr Trump also has trailed Mrs Clinton in fundraising. She reported raising nearly US$90 million (S$121 million) in July for her campaign and the Democratic Party, with more than half the donations coming from new donors.

Interviewed on the Fox Business television network, Mr Trump brushed off billionaire investor Warren Buffett's blistering critique of his business acumen.

Mr Buffett, chairman and chief executive officer of Berkshire Hathaway Inc, scorned Mr Trump's 1995 move to list Trump hotels and casino resorts on the New York Stock Exchange, saying the business lost money for the next decade and that a monkey would have outperformed Trump's company.

Mr Trump, who has said his business success qualifies him to lead the country, defended his record running his hotel and casino business in Atlantic City, New Jersey.

"I had great timing. I got out," after seven years, he told the network on Tuesday. "I took a lot of money out of Atlantic City, which is what I'm supposed to do. I'm a business person."