US bombers and Japanese fighters in joint Korea flyover

The US has flown two supersonic bombers to the Korean peninsula as a South Korean MP claimed the North had hacked into highly-classified military documents that detailed a "decapitation strike".

Two B1B bombers flew from the US territory of Guam to South Korea for drills overnight.

The US military said in a statement Japanese fighters had also joined the drill.

A South Korea military statement said the drills were part of a regular exercise to bolster and display the alliance between Seoul and Washington DC.

The South has taken an increasingly aggressive stance toward the North's belligerence amid back-and-forth threats of war between North Korea and US President Donald Trump.

North Korea's possession of secret war plans would require a major overhaul of how South Korea and its ally Washington would respond if there's another war on the Korean Peninsula.

An unusually aggressive approach to the North by Trump, which has included rhetoric hinting at US strikes and threatening the destruction of North Korea's leadership, has some South Koreans fearful that war is closer than at any time since the Korean War ended in 1953 in a shaky ceasefire.

Representative Lee Cheol-hee, a lawmaker for the ruling Democratic Party, cited unidentified defense officials as saying the hackers stole the US-South Korean war plans last year, according to news reports.

Among the classified plans allegedly stolen from the South were said to be blueprints for targeted attacks by Seoul and Washington to eliminate North Korean leader Kim Jong Un if a crisis breaks out or appears imminent.

The South's Yonhap news agency quoted Lee as saying that 235 gigabytes of military documents were taken. While nearly 80 per cent of the documents had not yet been identified, they reportedly included contingency plans for South Korean special forces and information on military facilities and power plants, it said.