Rolling out a foreign policy that he says will always put America first, Republican frontrunner Donald Trump has vowed that if elected president, he would make US allies in Europe and Asia take on more of the financial burden for their defence, or they would be left to defend themselves.

In a major speech, Mr Trump delivered a withering critique of President Barack Obama’s foreign policy, saying the Democratic president has let China take advantage of the US and has failed to defeat Islamic State (IS) militants.

Mr Trump pledged to “shake the rust off America’s foreign policy” and said he would seek better relations with China and Russia.

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

Mr Trump, who was also critical of policies of the last Republican US president, George W Bush, said he would use America’s strength sparingly.

He said he would build up the US military to keep pace with Chinese and Russian military programs but would use American armed forces only when absolutely necessary.

“I will not hesitate to deploy military force when there is no alternative,” Mr Trump said.

“But if America fights, it must fight to win.

“I will never send our finest into battle unless necessary — and will only do so if we have a plan for victory.”

Rancorous rhetoric set aside for teleprompter-aided speech

Mr Trump usually speaks in an off-the-cuff manner, but he delivered the speech with the aid of a teleprompter as he sought to make himself appealing to more Republican voters.

With US-Russian relations strained over numerous issues, including Moscow’s support for Syrian President Bashar al-Assad, Mr Trump said “an easing of tensions with Russia from a position of strength” is possible.

He also said he would use US economic leverage to persuade China to rein-in North Korea’s nuclear program.

“China respects strength and by letting them take advantage of us economically we have lost all their respect,” he said.

Mr Trump said he would call separate summits of NATO and Asian allies to discuss a “rebalancing” of the US financial commitment to their defence.

He was stern in charging that American allies have benefited from a US defence umbrella but have not paid their fair share.

“The countries we defend must pay for the cost of this defence,” he said.

“If not, the US must be prepared to let these countries defend themselves. We have no choice.”

Niels Annen, foreign policy spokesman of the Social Democrat party group in the German Parliament, criticised Mr Trump’s speech in a statement.

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