North Korea accused the United States on Monday of violating its airspace by conducting surveillance flights and warned that, while Pyongyang was exercising restraint, such flights may be shot down.

Provocative military actions by the United States were bringing the Korean peninsula closer to a nuclear conflict, said an unnamed spokesperson of North Korea's Ministry of National Defence in a statement carried by the official KCNA news agency.

The report also cited the use of US reconnaissance planes and drones and said Washington was escalating tensions by sending a nuclear submarine near the peninsula.

"There is no guarantee that such a shocking accident as the downing of the US Air Force strategic reconnaissance plane will not happen" in waters east of Korea, the spokesperson said.

The statement cited past incidents of the North shooting down or intercepting US aircraft at the border with South Korea and off the coast. North Korea has often complained about US surveillance flights near the peninsula.

There was no immediate response from the US military stationed in South Korea to a request for comment.

The moves by the United States to introduce strategic nuclear assets to the Korean peninsula is "the most undisguised nuclear blackmail" against North Korea and regional countries and presents a grave threat to peace, KCNA said.

"Whether the extreme situation, desired by nobody, is created or not on the Korean peninsula depends on the future action of the US, and if any sudden situation happens ... the US will be held totally accountable for it," it said.

US and South Korean forces have been conducting air and navy drills this year that involved a US aircraft carrier and heavy bombers. A US nuclear-powered cruise missile submarine also made a port call at Busan in South Korea last month.

The North's statement denounced what it called a US move to deploy a strategic nuclear submarine carrying nuclear warheads to the Korean peninsula for the first time since 1981.

In April, the leaders of South Korea and the United States agreed a US Navy nuclear-armed ballistic missile submarine will visit South Korea for the first time since the 1980s but no timetable has been given for such a visit. (Reuters)