Republican House Speaker Paul Ryan threw his support behind the party's presumptive presidential nominee Donald Trump on Thursday, a key step toward building unity ahead of the US general election in November.

"I'll be voting for @realDonaldTrump this fall," he tweeted. "I'm confident he will help turn the House GOP's agenda into laws."

The top-ranked Republican currently elected to public office, Mr Ryan dropped a bombshell last month, triggering soul-searching within the fractured party when he said he was "just not ready" to support Mr Trump as the flag bearer.

Mr Ryan is developing policy to serve as the basis for the Republican Party platform, a role that set him up for a major confrontation with a presidential nominee whose policies often oppose conservative orthodoxy.

Writing in his home state of Wisconsin's GazetteXtra, Mr Ryan said he would start introducing a series of policy proposals next week to "address the American people's top priorities."

"These plans are the result of months of work by House Republicans," he added.

"To enact these ideas, we need a Republican president willing to sign them into law," he said. "That's why, when he sealed the nomination, I could not offer my support for Donald Trump before discussing policies and basic principles."

However, following talks with the billionaire businessman last month under pressure from other party leaders, the House speaker said he is sure Mr Trump would implement his proposals and "help us turn the ideas in this agenda into laws to help improve people's lives."

Despite signals he would tone down his rhetoric to help build support among Republicans, Mr Trump has been especially combative this week, branding the political news media "dishonest" and one journalist "a sleaze."

Apparently addressing the shrinking pool of Republicans who vow to never support Mr Trump, Mr Ryan said likely Democratic nominee Hillary Clinton would be a worse choice.

"A Clinton White House would mean four more years of liberal cronyism and a government more out for itself than the people it serves," he wrote. "Quite simply, she represents all that our agenda aims to fix."

Elsewhere, Ms Clinton has said Mr Trump's foreign policy is dangerously incoherent.

Ms Clinton, who is still battling it out with Bernie Sanders for the Democrat nomination, said Mr Trump is unfit for the presidential office.

Cranking up what is already an acrimonious campaign, Ms Clinton said "he is temperamentally unfit to hold an office that requires knowledge, stability and immense responsibility."

Raising the nuclear issue, Ms Clinton said "this is not someone who should ever have the nuclear codes."