North Korea on Wednesday condemned training flights by nuclear-capable US B-52 bombers over the Korean peninsula as an "unpardonable provocation" and threatened military action if they continue.

The Pentagon says at least one B-52 has flown over South Korea in recent weeks as part of joint South Korea-US military exercises that Pyongyang has denounced as rehearsals for invasion.

"It is an unpardonable provocation," a North Korean foreign ministry spokesman said in a statement carried by the official Korean Central News Agency (KCNA).

"The US is introducing a strategic nuclear strike means to the Korean peninsula at a time when its situation is inching close to the brink of war," the spokesman said.

Military tensions on the Korean peninsula are at their highest level for years, with North Korea -- angered by UN sanctions imposed after its nuclear test last month -- threatening a second Korean War backed by nuclear weapons.

The foreign ministry said Pyongyang was closely watching the ongoing exercises and vowed a "strong military counteraction, should the strategic bomber make such a sortie to the peninsula again".

Pentagon spokesman George Little said Monday that a B-52 from Andersen Air Force base in Guam flew over South Korea on March 8.

South Korea's Yonhap news agency reported that another B-52 sortie was carried out Tuesday.

B-52s have taken part in annual exercises on the peninsula before, but Little said the Pentagon wanted to underline their use this time given the heightened tensions.

The flights should be seen as underscoring US commitment and capacity to defend Seoul against an attack from the North, Little said.

That message was echoed in Seoul on Monday by visiting Deputy Defense Secretary Ashton Carter, who promised to provide South Korea with every available military resource "offered by the US nuclear umbrella".