The United States said two nuclear-capable B-2 stealth bombers flew "deterrence" missions over South Korea on Thursday, defying apocalyptic threats of retribution from North Korea against ongoing war games.

The deployment of the stealth bombers was intended to send a potent message to Pyongyang about the US commitment to defending South Korea against any aggression, as military tensions on the Korean peninsula soar.

It came shortly after the North severed its last-remaining military hotline with South Korea and put its rocket units on combat status with a threat to target US bases in the Pacific region.

The two B-2s, from Whiteman Air Force Base in Missouri, flew the 13,000-mile (20,800-kilometre) round-trip in a "single continuous mission", dropping dummy ordnance on a target range in the South, the US military said in a statement.

"This... demonstrates the United States' ability to conduct long-range, precision strikes quickly and at will," the statement said.

The bombers were participating in South Korean-US military exercises that have incensed North Korea, which has threatened to unleash a second Korean War and launch pre-emptive nuclear strikes on South Korea and the US mainland.

"The United States is steadfast in its alliance commitment to the defense of the Republic of Korea," the US statement said.

Earlier Thursday, US Defense Secretary Chuck Hagel told his South Korean counterpart that Seoul could rely on all the military protection the United States has to offer -- nuclear, conventional and missile defence.

The US and South Korean militaries signed a new pact last week, providing for a joint military response to even low-level provocation by North Korea.

Yonhap news agency quoted a Seoul military official as saying Washington had recently approved the sale of US "bunker buster" bombs to South Korea -- capable of targeting the North's extensive underground military facilities.

The use of the stealth bombers is sure to prompt a fresh outcry from Pyongyang, which has already denounced the use of US B-52 bombers in the joint exercises.

The drills are held annually and are regularly condemned by Pyongyang as provocative rehearsals for an invasion.

Their staging this year came as tensions were already riding high following the North's long-range rocket launch in December and its nuclear test last month.

While most analysts have dismissed the bulk of the North Korean threats as rhetorical bluster, there are concerns that even a minor incident could swiftly escalate in such a volatile environment.

In its latest protest at the military drills, North Korea announced Wednesday that it was severing its military hotline with the South, saying it was no longer needed given that "war may break out any moment".

The North has severed the hotline before, most recently in March 2009. In that case the line was reconnected less than two weeks later.

Several weeks ago North Korea cut a Red Cross hotline that had been used for government-to-government communications.

Among other things, the military hotline was used on a daily basis to organise movement in and out of the Kaesong industrial complex -- a joint South-North Korean venture established in 2004.

The South Koreans used the line to give the North the names of those seeking entry to Kaesong, guaranteeing their safety as they crossed one of the world's most heavily militarised borders.

The crossing was operating normally Thursday, officials said, adding that they had used a civilian link to get the names to the North Korean guards.

North Korea has always been wary of allowing crises in inter-Korean relations to affect the zone, which lies 10 kilometres (six miles) inside its border and is a crucial hard-currency earner for the communist state.