WASHINGTON • Sixty Republican foreign policy veterans released a letter pledging to oppose Mr Donald Trump, saying his proposals would undermine US security, in the latest sign of fissures between the Republican presidential front runner and the party establishment.

"Mr Trump's own statements lead us to conclude that as president, he would use the authority of his office to act in ways that make America less safe, and which would diminish our standing in the world," the letter released on Wednesday said.

The signatories include former World Bank president and deputy secretary of state Robert Zoellick, former US homeland security secretary Michael Chertoff and Mr Dov Zakheim, a top Pentagon official under President George W. Bush.

They represent both centrist Republican foreign policy circles and neo-conservatives who wielded clout during Mr Bush's tenure.

Billionaire businessman Trump's big wins on Tuesday intensified moves by the party's establishment to derail his path to the nomination.

Mr Bryan McGrath, an adviser to Mr Mitt Romney's unsuccessful 2012 presidential campaign who helped organise the effort, said that at least two people declined to sign the letter, citing concerns that it would only fuel Mr Trump's campaign theme of being an anti-Washington candidate opposed by the establishment.

The letter, posted on a blog site called War on the Rocks, rejects numerous Trump foreign policy statements, including his anti-Muslim comments and his demand that Mexico fund a wall to control illegal immigration.

Mr Kurt Volker, who was a permanent representative to Nato under Mr Bush, said he did not sign the letter out of concern it could end up backfiring.

"My concern is that... he would use that as a tool, saying, 'Here is the establishment. More of the same. They are afraid of me. I can do better.' He would actually use it as a bragging right."

REUTERS