More than 70 US Republican foreign policy veterans​ have released a letter lashing out at Donald Trump for his proposals during the presidential campaign, vowing to oppose him.

"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 says.

The signatories say the proposals by the Republican frontrunner would undermine US security. The signatories have also expressed concern that Trump’s use of presidential power against critics would threaten civil liberty in the US.

"Furthermore, his expansive view of how presidential power should be wielded against his detractors poses a distinct threat to civil liberty in the United States," it says.

The signatories include Robert Zoellick, a former World Bank president and deputy secretary of state; former US Homeland Security Secretary Michael Chertoff; and Dov Zakheim, a senior Pentagon official under the George W. Bush administration.

The signatories have rejected Trump’s statement that Muslims should be banned from entering the country.

They have also slammed Trump for his demand that Mexico funds a wall to control illegal immigration across the American border.

The letter is the latest sign of cracks between Trump and the Republican Party’s leading figures.

Trump, who has never held elected office, is leading the Republican presidential primary field, despite the fact that his campaign has been marked by controversial statements, including with disparaging remarks about Mexican immigrants and Muslims.

Trump says he would deport 11 million undocumented workers from the United States and would establish a “deportation force” for this purpose.

He has also promised to expel undocumented immigrants and build a wall on the US-Mexico border. In addition, he has proposed repealing the constitutional right to citizenship of anyone born on US soil.

Trump has also created a furor in the US and around the world by proposing a “total and complete shutdown of Muslims” entering the United States, following a mass shooting in California.

The New York real-estate mogul has also called for a database to track Muslims across the United States, and he has also said that the US would have "absolutely no choice" but to close down mosques.

Trump made a further step toward a nominee for the November presidential election by winning in seven states on Super Tuesday.