Donald Trump is expected to announce his vice presidential pick before he goes to the party's national convention in Cleveland next week, his campaign manager said on Friday.

The presumptive nominee had said Thursday evening that he was postponing his vice presidential announcement after an attack in the French town of Nice killed dozens of people.

"Another horrific attack, this time in Nice, France. Many dead and injured. When will we learn? It is only getting worse," Trump tweeted after learning that a truck had plowed into a crowd of people watching fireworks for Bastille Day.

"In light of the horrible attack in Nice, France, I have postponed tomorrow's news conference concerning my Vice Presidential announcement," Trump said in a second tweet.

Trump's campaign chairman Paul Manafort said in television interviews on Friday that he expects Trump to announce a new day and time for revealing his pick for vice president ahead of the Republican National Convention, which begins Monday.

Trump was slated to announce his running mate at a news conference in New York City on Friday, and sources told Reuters he had picked Indiana Gov. Mike Pence.