The US believes North Korea is testing a new intercontinental ballistic missile (ICBM) in what the Biden administration called a “serious escalation” that would trigger more sanctions.

Pyongyang conducted two recent missile launches which it said were ultimately intended for putting satellites into space. After scrutinising them, however, US intelligence has assessed that the real intention was to test parts of the new ICBM.

“Based on analysis of these launches, the United States government has concluded that these launches involved a new ICBM system that the DPRK is developing,” John Kirby, the Pentagon spokesman, said.

“This is a serious escalation by the DPRK.”

The US believes the ICBM being tested was first displayed by the Pyongyang regime in the Workers’ party parade on 10 October 2020, and then again at a defence exhibition a year later.

“The purpose of these tests, which did not demonstrate ICBM range, was likely to evaluate this new system before conducting a test at full range in the future, potentially disguised as a space launch,” Kirby said.

The US made its assessment in conjunction with Japan and South Korea, a senior US official added, and would share the conclusions with other allies and partners, including the United Nations.

Japan’s defence ministry condemned the tests. “North Korea’s recent actions, including these missile launches that make the situation more tense, threaten the peace and security of our country, region and the international community and are totally unacceptable,” it said in a statement.

Japan is particularly concerned about the North’s recent return to missile testing. Two intermediate-range missiles flew over its territory in the space of a fortnight in 2017, triggering alerts warning people to take cover.

US forces have stepped up their monitoring and intelligence collection in the region, and new sanctions are to be introduced on Friday aimed at further restricting North Korea’s access to advanced technology that it could use in its weapons programmes.

“There will be a range of further actions in the coming days,” the US official said.

The White House says that Joe Biden is open to diplomacy and would be prepared to meet the North Korean leader, Kim Jong-un, “when there is a serious agreement on the table”.

The Biden administration believes any such agreement would need to be preceded by working-level negotiations, as the experience of Donald Trump’s meetings with Kim suggested that “summits alone are no guarantee of progress,” the senior official said, adding: “The DPRK continues to not respond.

“While the door remains open to diplomacy, the United States will take all necessary measures to ensure the security of the American homeland and our allies,” the official said.

North Korean state media on Friday reported that Kim had ordered officials to expand a satellite launch facility to fire a variety of rockets that could carry multi-purpose satellites.

“He stressed that it is the noble duty of our party and space scientists and technicians … to turn the launching ground, associated with our state’s great dream and ambition for a space power, into an ultramodern advanced base,” the KCNA news agency quoted Kim as saying during a visit to the site.

Pyongyang insists its satellite launches are part of its peaceful space programme, but the UN believes they are used to try out new missile technology.

North Korea conducted three ICBM tests in 2017, and has carried out a total of six nuclear tests, before suspending both types of testing before the summits with Trump.

In January, Pyongyang said it would reconsider “restarting all temporarily suspended activities”, and analysts have seen signs of construction at North Korea’s Punggye-ri nuclear test site for the first time since its closure and dismantlement in the spring of 2018.

Some experts believe North Korea could launch a spy satellite or a test an ICBM in April to coincide with the 110th anniversary of the birth of the country’s founder, and Kim’s grandfather, Kim Il-sung.