The United Nations atomic agency has expressed concern about the nuclear programme of North Korea, saying satellite imagery suggests that the country is moving ahead with its nuclear efforts.

In an interview published on Thursday, Yukiya Amano, head of the International Atomic Energy Agency (IAEA), told German daily newspaper Sueddeutsche Zeitung that the agency has "concrete information that the nuclear programme is going ahead just as North Korea is publicly stating".

The IAEA is basing its conclusions on satellite surveillance of the communist country, as its inspectors were evicted from North Korea in 2009.

READ MORE: Why is North Korea testing nuclear weapons?

Inspectors have since continued to collect and evaluate information from satellite imagery, open-source and trade-related information.

"All available clues show that North Korea is making progress, and that worries us," Amano added, pointing out that the security risks would apply beyond the region.

On Thursday, US-based analysts also said that satellite images provided to them indicated that activity had resumed at North Korea's nuclear test site.

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

Pyongyang says it has conducted five nuclear weapons tests since 2006. Two of them are supposed to have taken place last year. Furthermore, the country is repeatedly breaching UN resolutions and testing various missiles.

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.

US President Donald Trump has been trying to increase pressure on North Korea and is counting on China's support to do so.

Trump said that all options were on the table in the case of Pyongyang, but lately toned down his statements, mentioning he would even be inclined to meet North Korea's leader, Kim Jong-un, under certain conditions.

Amid tensions and belligerent rhetoric from North Korea, the US test-fired an unarmed ballistic missile from an airbase in California.

READ MORE: IAEA chief: North Korea continues nuclear test efforts

The Minuteman 3 is an intercontinental missile capable of transporting nuclear warheads. The US air force has about 450 such weapons, part of its nuclear arsenal.