The US military would find itself outnumbered and under-supplied if war broke out with North Korea, a former top commander in the region has reportedly claimed.

American troops “are vastly outnumbered by North Korean forces, as well as [allied] forces that will conduct the overwhelming majority of the fighting”, according to retired Lt Gen Jan-Marc Jouas.

They would also face problems in getting reinforcements or new supplies, he claimed, saying fresh troops “may well find their bases subject to attack by conventional or chemical weapons, which will further delay their entry into the war”.

In a letter to members of Congress, obtained by Newsweek, Lt Gen Jouas said even limited military action against Kim Jong-un’s regime would be likely to precipitate a full-scale war, yet would probably not destroy Pyongyang’s nuclear capability totally.

He added: “An enormous casualty and evacuee crisis will develop and include over 100,000 non-combatant Americans, many of who will turn to US forces to get them off the peninsula.”

The use of nuclear weapons by Pyongyang could not be ruled out, he said. Lt Gen Jouas was deputy commander of US Forces Korea from January 2012 until his retirement in early 201.

US personnel in South Korea number about 28,500, Lt Gen Jouas said, though news reports place recent figures both higher and lower.

H R McMaster calls North Korea a threat 'to the entire world'

An all-out war with North Korea would be bloody in the extreme, experts believe, as the country has huge amounts of ordnance aimed at Seoul, the South’s capital, which is just a few dozen miles from the demilitarised zone.

US Defence Secretary James Mattis has said the results of conflict would be “catastrophic”.

According to the Council on Foreign Relations (CFR), a US think-tank, Pyongyang has some 1.1 million personnel in its armed forces.

The CFR added in a recent analysis: “Although Pyongyang is outspent by its neighbours and adversaries in dollar-to-dollar comparisons and defence experts say it operates with ageing equipment and technology, the regime’s forward-deployed military position and missiles aimed at Seoul ensure that Pyongyang’s conventional capabilities remain a constant threat to its southern neighbour.”

Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Show all 6 1 /6 Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Photos released by North Korea show Kim Jong-un talking to subordinates next to a device thought to be the new thermonuclear weapon. There is no way of independently verifying the pictures STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korea claims it has successfully tested an advanced hydrogen bomb which could be loaded onto an intercontinental ballistic missile AFP/Getty Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A diagram on the wall behind Mr Kim shows a bomb mounted inside a cone STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korean leader Kim Jong-Un (C) attending a photo session with participants of the fourth conference of active secretaries of primary organisations of the youth league of the Korean People's Army (KPA) in Pyongyang STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters

Lt Gen Jouas’ letter comes at a time of heightened tension between the US and North Korea, and as US President Donald Trump is in the middle of a tour of the Asia-Pacific.

However, Mr Trump appears to have introduced a hint of a softer tone to his statements on North Korea. In Seoul earlier this week he said: “It makes sense for North Korea to come to the table and make a deal that is good for the people of North Korea and for the world.”