US President-elect Donald Trump says he will nominate retired General James Mattis to be his defence secretary, making the announcement during a post-election victory rally in Cincinnati.

Key points: US President-elect names retired 66-year-old General as his defence secretary nominee

US President-elect names retired 66-year-old General as his defence secretary nominee Congress will have to bypass law barring retired military officers from the role

Congress will have to bypass law barring retired military officers from the role General Mattis once said it was "fun to shoot people"

General Mattis, 66, is a Marine Corps general who retired in 2013 after serving as the commander of the US Central Command.

His selection raises questions about increased military influence in a job designed to ensure civilian control of the armed forces.

The concerns revolve around whether a recently retired service member would rely more on military solutions to international problems, rather than a broader, more diplomatic approach.

For General Mattis to be confirmed, Congress would first have to approve legislation bypassing a law that bars retired military officers from becoming defence secretary within seven years of leaving active duty.

General Mattis has a reputation as a battle-hardened, tough-talking Marine who was entrusted with some of the most challenging commands in the US military.

In a tweet on Monday, Mr Trump referred to General Mattis by his nickname "Mad Dog".

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General Mattis would be only the second retired general to serve as defence secretary, the first being George C Marshall in 1950-51 during the Korean War.

"We are going to appoint 'Mad Dog' Mattis as our secretary of defence," Mr Trump told a cheering crowd in Ohio.

"But we're not announcing it until Monday, so don't tell anyone. Mad Dog. He is great."

General Marshall was a much different figure, having previously served as US secretary of state and playing a key role in creating closer ties with western Europe after World War II.

The only previous time an exception was made to the law barring someone from becoming defence secretary within seven years of leaving active duty was for General Marshall.

Although his record in combat and his credentials as a senior commander are widely admired, General Mattis has little experience in the diplomatic aspects of the job of a secretary of Defence.

Mattis a 'defence intellectual'

Richard Fontaine, president of the Centre for a New American Security, described General Mattis as a defence intellectual and as a military leader who distinguished himself in combat.

"He knows the Middle East, South Asia, NATO and other areas and has evinced both a nuanced approach to the wars we're in and an appreciation for the importance of allies," Mr Fontaine said in an email exchange.

"If he were to get the nomination, I suspect that he could attract a number of very talented people to work with him."

But General Mattis has not been immune to controversy. He was criticised for remarking in 2005 that he enjoyed shooting people.

He also drew more recent scrutiny for his involvement with the embattled biotech company Theranos, where he serves on the board.

Born in Pullman, Washington, General Mattis enlisted in the Marine Corps in 1969, later earning a history degree from Central Washington University. He was commissioned as an officer in 1972. As a lieutenant colonel, he led an assault battalion into Kuwait during the first US war with Iraq in 1991.

As head of the Central Command from 2010 until his retirement in 2013, he was in charge of both the wars in Iraq and Afghanistan.

In the aftermath of the 9/11 attacks, General Mattis commanded the Marines who launched an early assault into Afghanistan and established a US foothold in the Taliban heartland.

As the first wave of Marines moved toward Kandahar, he declared: "The Marines have landed, and now we own a piece of Afghanistan."

Two years later, he helped lead the invasion into Iraq in 2003 as the two-star commander of the 1st Marine Division.

'Fun to shoot people' comments raised eyebrows

General Mattis enlisted with the Marine Corps in 1969. ( Reuters: Dave Martin )

In 2005, he raised eyebrows when he told a San Diego forum that it was "fun to shoot some people".

According to a recording of General Mattis' remarks, he said, "Actually, it's a lot of fun to fight. You know, it's a hell of a hoot. ... It's fun to shoot some people. I'll be right up front with you, I like brawling."

"You go into Afghanistan, you got guys who slap women around for five years because they didn't wear a veil," General Mattis continued.

"You know, guys like that ain't got no manhood left anyway. So it's a hell of a lot of fun to shoot them."

He was counselled to choose his words more carefully.

A year later, General Mattis came under scrutiny during one of the more high-profile criminal investigations of the Iraq war, the shooting deaths of 24 Iraqis by Marines.

The Iraqis, who included unarmed women and children, were killed by Marines in the town of Haditha after one of their comrades was killed by a roadside bomb.

Eight Marines were charged in connection with the killings - four enlisted men were charged with unpremeditated murder and four officers who weren't there at the time were accused of failures in investigating and reporting the deaths.

As commander of the accused Marines' parent unit, the 1st Marine Expeditionary Force, General Mattis ultimately dismissed charges against most of the Marines.

As a top Marine general, General Mattis pushed for the military to adopt blood-testing technology developed by Theranos.

AP