North Korea has warned that it will bolster its nuclear programme and take unspecified counter-measures unless the US eases diplomatic pressure on it.
The Foreign Ministry accused the US of persisting in "moves to ratchet up sanctions", state media reported.
Last week world leaders condemned the North's failed rocket launch last month and warned that more nuclear tests or launches would bring tougher sanctions.
Satellite images show activity at the North's nuclear test site, experts say.
"Images taken by DigitalGlobe and GeoEye in the past month show mining carts and excavation equipment present at the active tunneling site, yielding the largest amount of spoil - or debris recovered from inside the tunnel - around the tunnel entrance to date," James Hardy of Jane's Defence Weekly told South Korea's Yonhap News Agency.
He said a 9 May DigitalGlobe image showed new road networks within the spoil piles at the Punggye-ri nuclear test site in North Korea's north-east, as well as "carts and a vehicle in the site operations facility".
The North reiterated that the rocket had been an attempt to put a satellite into orbit for peaceful purposes, but critics said it was a disguised test of missile technology banned under UN resolutions.
In a statement on Tuesday, North Korea said it would "continue to legitimately" push for its right "to launch satellites".
"We had access to nuclear deterrence for self-defence because of the hostile policy of the US to stifle the DPRK by force and we will expand and bolster it non-stop as long as this hostile policy goes on," the spokesman said, state news agency KCNA reported.
"If the US persists in its moves to ratchet up sanctions and pressure upon us despite our peace-loving efforts, we will be left with no option but to take counter-measures for self-defence."
The statement comes amid increased tension in the region after the North's long-range rocket launch last month.
The launch came shortly after Washington and Pyongyang agreed on a deal to swap food aid for a moratorium on nuclear and ballistic missile activities.
Glyn Davies, US special envoy for North Korea policy, said on Monday that any nuclear test from North Korea would be "a serious miscalculation".
Speaking after talks with Japanese and South Korean officials in Seoul, he said that any such test would lead to a "swift and sure response" from the region.----
In: World News, Politics
Tags: north korea, threats, US, diplomatic, sanctions, nuclear, deterrent
Location: Korea (North) (load item map)
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