Republican White House candidate Donald Trump on Friday formally announced Mike Pence, the governor of Indiana, as his choice for vice president, ending weeks of intense speculation and putting a conservative stalwart on the ticket.

Pence, 57, only emerged on the short list in recent weeks, after better-known party heavyweights including vanquished presidential contenders like John Kasich or Ted Cruz either declined to be considered or appeared to not make the cut.

In a whirlwind 72 hours before the announcement, Trump met privately with Pence and his family, and also with two other men on the short list: New Jersey Governor Chris Christie and former House speaker Newt Gingrich.

Pence brings all-important executive and legislative experience to the ticket, which could assuage concerns among conservatives that Trump lacks governing knowledge or the skills to navigate Congress.

"I am pleased to announce that I have chosen Governor Mike Pence as my Vice Presidential running mate," Trump posted on Twitter.

The provocative billionaire scheduled a joint appearance with Pence in New York on Saturday at 11:00 am (1500 GMT).

Trump had earlier stated he would publicly reveal his pick at a Friday press conference, but postponed it after Thursday's truck attack in the French city of Nice that left at least 84 people dead.

Pence spent 12 years in the House of Representatives, many of those on the House Foreign Affairs Committee, headed up a conservative caucus, and also spent time in House leadership.

He has been Indiana's governor since January 2013.

Pence is also seen as having a cool-headed tone and steady hand that could counter the impulsive, often brash Trump, and his social conservatism could allay fears about Trump's views on issues such as abortion.

"Honored to join @realDonaldTrump and work to make America great again," Pence said, using Trump's campaign slogan.

Presumptive Democratic presidential nominee Hillary Clinton's campaign reacted swiftly to the Pence announcement, calling him "the most extreme pick in a generation."

"Donald Trump has doubled down on some of his most disturbing beliefs by choosing an incredibly divisive and unpopular running mate known for supporting discriminatory politics and failed economic policies that favor millionaires and corporations over working families," Clinton campaign chairman John Podesta said in a statement.

Trump had stayed mum on his choice during television interviews Thursday in which he explained he was postponing his announcement out of respect for the victims of the Nice attack.

Even as US media cited sources saying Trump had decided on Pence, Trump fueled suspicion that he was having second thoughts when he said Thursday: "I haven't made my final, final decision."

He stressed he had "great respect" for Pence, as well as for Gingrich and Christie, who are more blunt but also more unpredictable than Pence.

Gingrich unleashed harsh new rhetoric after the Nice attack, reflecting Trump's controversial anti-Muslim position.

"Western civilization is in a war," Gingrich told Fox on Thursday.

"We should frankly test every person from here who is of a Muslim background and if they believe in sharia (law) they should be deported. It is incompatible with Western civilization."

Pence split openly with Trump last December over his divisive anti-Muslim rhetoric, signalling he might push back against Trump on certain issues if they were in the White House.

"Calls to ban Muslims from entering the U.S. are offensive and unconstitutional," Pence tweeted at the time.

Seen as disciplined and relatively discreet, the Christian conservative Pence was apparently the favorite of Trump's children, who exercise much influence with their political neophyte father as he campaigns for president.

Pence earned plaudits from fellow Republicans.

"Great pick. @mike_pence is rock solid," tweeted former 2016 presidential candidate Marco Rubio, a US senator from Florida.

University of Virginia political professor Larry Sabato said Pence is struggling with low approval ratings in Indiana and is a virtual unknown on the national stage.

Trump was wise not to have picked the more explosive Gingrich or Christie, Sabato said.

"Trump is already a box of dynamite. Why do you need to add a stick of dynamite to a box of dynamite?" he told AFP.

Trump's announcement comes three days before the Republican National Convention opens in Cleveland, Ohio, where Trump will officially become the party's presidential nominee.

Meanwhile, a longstanding effort to deny Trump the nomination appears to have failed, when the convention's rules committee refused to allow a clause that would have let delegates vote their "conscience" at the convention.

"#NeverTrump is never more," Trump gloated on Twitter. "They were crushed last night in Cleveland at Rules Committee by a vote of 87-12."