Satellite images indicate activity has resumed at North Korea's nuclear test site, US-based analysts said, as tensions remain high over fears of a sixth atomic test by the reclusive state.

Images of the Punggye-ri site captured on April 25 appear to show workers pumping out water at a tunnel believed to have been prepared for an upcoming nuclear test, monitoring group 38 North said.

It also noted that a number of personnel were seen throughout the facility.

"It is unclear if this activity indicates that a nuclear test has been cancelled, the facility is in stand-by mode or that a test is imminent," researchers from the monitoring group at John Hopkins University's US-Korea Institute said.

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Workers were also observed playing volleyball at the guard barracks and two other areas at the site in satellite pictures taken on April 19 and 21.

38 North said the latest images were "unusual and almost assuredly a component of an overall North Korean deception and propaganda effort" and the result of media reporting on earlier images showing renewed activity at the site, which consists of a complex of tunnels and testing infrastructure in the mountains in the northeast of the country.

North Korea has carried out five nuclear tests in the past 11 years and is widely believed to be making progress on building a missile capable of delivering a warhead to the continental US.

On Monday its foreign ministry warned that the country was prepared to carry out a nuclear test "at any time and at any location" set by its leadership.

North Korea will continue bolstering its "pre-emptive nuclear attack" capabilities unless the US scraps its hostile policies, a statement carried by the state-run KCNA news agency said.

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The announcement came as US officials said the US military's missile defence system in South Korea had reached an initial operating capability to defend against North Korean missiles.

38 North said last month that Punggye-ri was "primed and ready" to conduct a test, amid mounting speculation that Pyongyang would act to coincide with major anniversaries.

US President Donald Trump said this week he would be "honoured" to meet North Korean leader Kim Jong-un under the right conditions, dialling down earlier threats of military action.

The White House promptly told the press that conditions were not right yet.

Washington is now exploring options at the UN Security Council to ramp up pressure on North Korea, with diplomats saying it was in discussion with China on possible sanctions.

Over the past 11 years, the Security Council has imposed six sets of sanctions on Pyongyang, including imposing a cap on coal exports among other measures in November.