Republican presidential candidate Donald Trump has detailed plans to launch a new United States military build-up if he wins November’s election.

In a prepared speech at a conservative club in Philadelphia, Mr Trump unveiled plans to grow the army by 50,000 to 540,000 soldiers, increase the size of the US Marine Corps, establish a new missile defence system, and build new ships and warplanes.

Echoing late president Ronald Reagan, he said he would adopt a policy of “peace through strength”.

“We’ll be one people, under one God, saluting one American flag,” he said.

The pledge came ahead of a televised defence policy forum when Mr Trump and his Democratic opponent Hillary Clinton will compete to show who would be more competent at leading the armed forces as commander-in-chief.

Buoyed by polls showing him regaining some ground on Mrs Clinton, Mr Trump portrayed himself in the address as a defender of traditional Republican values on national security.

He said he would ask Pentagon leaders to present a plan within 30 days to defeat and destroy the Islamic State group if he won the November 8 election.

“I am proposing a new foreign policy focused on advancing America’s core national interests, promoting regional stability, and producing an easing of tensions in the world,” he said.

“This will require rethinking the failed policies.”

Donald Trump plans to ramp up US military with hundreds of new ships, planes and submarines. Photo: Getty

The New York businessman gave the speech in measured tones and offered more policy detail than he often does at campaign rallies.

He called for hundreds more new US ships, planes and submarines, and vowed to train thousands more combat troops as well as developing a “state of the art” missile defence system, starting with modernising 22 Navy cruisers at a cost of about $US220 million ($287 million) apiece.

He told supporters at the invite-only event in the Union League of Philadelphia that America was under threat like never before from foes like radical Islamists, North Korea and China.

“Our adversaries are chomping at the bit,” he said, blaming President Barack Obama and his former secretary of state Mrs Clinton for allowing the United States to lose global influence.

He accused Mr Obama of wanting to reduce the size of the Army to 450,000 troops.

– ABC.