Satellite images show that a ''major upgrade'' is underway at North Korea's rocket launch site, says a United States institute.

美国一家机构称，卫星照片显示朝鲜火箭发射基地正在进行一场“大规模改进”。

Work at the Musudan-ri site showed "rapid progress" since mid-2011, the analysis said.

The report came as Pyongyang accused Washington of trying to ''incite¹（煽动，刺激） confrontation²'' over speculation³ it may carry out a third nuclear test.

North Korea ''never envisaged⁴'' such an act, said a foreign ministry⁶ spokesman.

The remarks followed a US warning that a nuclear test would lead to a "swift and sure response" from the region.

Glyn Davis, the US special envoy⁷ for North Korea policies, said on Monday that any such move by Pyongyang would be "a serious miscalculation".
Construction 'pace'

The satellite images, taken on 29 April of the Tonghae Satellite Launching Ground (also known as Musudan-ri) were analysed by the 38north website of the US-Korea institute at the Paul H. Nitze School of Advanced International Studies (SAIS) at Johns Hopkins University.

Citing fast progress on upgrading work, the analysis said: ''At the current pace of construction, the facilities should be operational by 2016-17.''

It also noted⁸ similarities between the North Korean buildings and those at Iran's Semnan Missile and Space Center.

''Nevertheless, while the two countries have a long history of missile co-operation, it is too soon to tell whether that co-operation extends to the design and construction of this facility or the new long-range liquid-fuelled rocket,'' it added.

Pyongyang's missile and nuclear ambitions faced increased scrutiny⁹ in recent months, following the death of Kim Jong-il last December and installation of his son Kim Jong-un.

In the wake of North Korea's failed rocket launch last month, South Korea also reported that preparations for a third nuclear test appeared to be under way.

In a statement on Tuesday, North Korea hit out at US comments on the possible test, saying the country had told the US that it was ''restraining'' itself.

''From the beginning, we did not envisage⁵ such a military measure as a nuclear test as we planned to launch a scientific and technical satellite for peaceful purposes,'' the ministry spokesman said in a statement carried by KCNA news agency.