Rolling out an "America first" foreign policy, Republican front-runner Donald Trump has vowed that if he were elected president, US allies in Europe and Asia would have to fend for themselves if they did not pay more for the US defence umbrella.

Trump's speech, delivered with a teleprompter in a staid Washington hotel ballroom, was an attempt to show he can be more presidential and move past the rancorous rhetoric that he routinely uses on the campaign trail.

"It is time to shake the rust off of America's foreign policy," he said in a speech that savaged the foreign policy of Democratic President Barack Obama as a disaster.

The New York billionaire spoke the day after victories in five Northeastern states that moved him closer to capturing the Republican Party presidential nomination for the November 8 election.

But the message Trump delivered appeared contradictory at times and was largely devoid of details.

He spoke of building up the US military as a deterrent to US adversaries, but said American allies in Europe and Asia would have to pay more for US defences provided by Washington.

While issuing that stern message on paying for defence, Trump said the US under his leadership would be "a reliable friend and ally again."

Trump was sharply critical of immediate past presidents, both Republican and Democratic, for getting Americans involved in military conflicts abroad, but said the US may well need to use force to defeat Islamic State militants.

In perhaps his most specific policy prescription, Trump said he would organise two summits, for Nato allies and Asian allies, to discuss "rebalancing" the alliances to ease the financial burden to the US.

"The countries we defend must pay for the cost of this defence. If not, the US must be prepared to let these countries defend themselves. We have no choice," Trump said.

Trump's speech was panned by his nearest rival for the Republican nomination, US Senator Ted Cruz of Texas, who said in a tweet: "Donald's speech is the most dramatic evidence thus far that he fails the presidential test."

Also critical was US Senator Lindsey Graham of South Carolina, a hawkish Republican who was a presidential candidate until dropping out early this year.

"It's isolationism surrounded by disconnected thought, demonstrates lack of understanding threats we face," Graham wrote on Twitter. "Not sure who is advising Trump on foreign policy but I can understand why he's not revealing their names."

Trump has gone from outsider last year to party front-runner with a plain-talking campaign that has often appealed to working-class voters who feel let down by globalisation, free trade and the decline of American manufacturing in recent decades.

His foreign policy speech echoed that populist message - depicting a need to ease the US financial burden overseas, focus more on nation-building at home and make sure American companies pay a price for outsourcing jobs to countries where labor is cheaper.

"My foreign policy will always put the interests of the American people, and American security, above all else. That will be the foundation of every decision that I will make," Trump said. "'America first' will be the major and overriding theme of my administration."

GERMANY REACTS

Niels Annen, foreign policy spokesman of the Social Democrat party group in the German parliament, criticised Trump's speech in a statement to Reuters.

"Trump's campaign gives America's friends a feeling of uncertainty. At least he seems to have realized that as a candidate for the White House he would have to have a coherent foreign policy concept," Annen said.

Political pundits quickly pointed out that "America First" was a slogan first developed by famed aviator Charles Lindbergh, who pushed a non-interventionist policy before the US entry in World War Two.

If Trump wins the nomination and runs against Democratic front-runner Hillary Clinton, he will be up against a former secretary of state steeped in foreign policy.

Clinton adviser Madeleine Albright, herself a former secretary of state, said Trump's address was centered on "simplistic slogans and contradictions."

"I've got to say, I've listened to a lot of foreign policy speeches over the years, and have given a lot myself, and I was hoping it would make some sense," Albright said.

'SELLING A DREAM'

Jon Alterman, director of the Middle East program at the Center for Strategic and International Studies think tank in Washington, said Trump's speech did not contain enough details to call it a strategy.

"Ultimately, he's selling a dream and he's still not offering a plan. He's representing the sales office, but he's given no clue of who will be the architect and who will do the construction," he said.

Trump did not stray from the ideas that have helped put him close to winning the Republican presidential nomination. He would build up the US military but would use it only when necessary and would be unpredictable in using force, as in the case of Islamic State.

"Their days are numbered. I won't tell them where and I won't tell them how. We must, as a nation, be strong and resilient. They're going to be gone," he said.

Trump said US leaders had allowed the American nuclear arsenal to atrophy and that it was in desperate need of modernisation. The Obama administration is pushing a $1 trillion refurbishment programme over the next 30 years.

Trump was particularly withering in his critique of Obama's foreign policy, saying the president had let China take advantage of the US and not been able to persuade Beijing to rein in North Korea.

"We have the leverage. We have the power over China, economic power," he said.

According to social media analytics firm Zoomph, Twitter reaction to the speech was more positive than negative, with roughly 100 posts per minute mentioning the Republican front-runner.

Walid Phares, a Trump foreign policy adviser, said Trump received input for his address from a variety of advisers, such as US Senator Jeff Sessions of Alabama. - Reuters