The U.S. has imposed sanctions on North Korea's primary foreign exchange bank and four individuals, amid heightened tensions over its nuclear ambitions, BBC News reported.

The move followed a UN vote last week to expand sanctions on Pyongyang after its February nuclear test. It comes amid escalating rhetoric from the North over the sanctions and U.S.-South Korean exercises now underway.

On Monday, March 11, North Korea cut the hotline between the two Koreas and said the Korean War armistice was "invalid".

Spokesman Jay Carney said the White House was "certainly concerned by North Korea's bellicose rhetoric". "The threats that they have been making follow a pattern designed to raise tension and intimidate others."

A spokesman for UN Secretary-General Ban Ki-moon, meanwhile, said the terms of the Korean War armistice "do not allow either side unilaterally to free themselves from it''.

The two Koreas remain technically at war because the armistice, not a peace treaty, ended the 1950-53 conflict. Although the hotline has been cut, other channels of communication exist between them.

On Tuesday, a spokesman for South Korean President Park Geun-hye said that her first overseas trip since taking office would be to the US in May, taking into account "the recent Korean Peninsula situation".

North Korea conducted its third nuclear test on 12 February, a move that followed an apparently successful launch in December of a three-stage rocket, seen as a banned test of missile technology.

Regional neighbors and the US fear Pyongyang is working to build a nuclear warhead small enough to put on a missile, but believe it does not yet have the capabilities to do so.

Last week, the UN Security Council passed a resolution expanding sanctions on North Korea's financial interests in response to the nuclear test.

To these the U.S. Treasury on Monday added sanctions on the Foreign Trade Bank (FTB) of North Korea and Paek Se-bong, the chairman of the North's Second Economic Committee, which it said oversaw production of ballistic missiles.

The U.S. State Department also added three North Korean officials it said had links to the communist country's proliferation activities.

The sanctions ban any U.S. individual or entity from transactions with those named.

In a speech, U.S. National Security Advisor Tom Donilon said North Korea should "change course" to obtain the aid and respect it sought.