The US government says North Korea could be ready to conduct its seventh nuclear test at a site in the country's northeast as early as this month.

A State Department spokesperson expressed the view during a phone briefing on Friday.

Jalina Porter said the US has shared the information with its allies and partners, and will continue to closely coordinate with them.

Porter said the US will build on the cooperation when President Joe Biden travels to South Korea and Japan later this month. She said the trip's purpose is "to strengthen our alliances and demonstrate our commitment to their security is ironclad."

The Center for Strategic and International Studies, a US think tank, has reported that satellite images of the Punggye-ri nuclear test facility collected on April 25 indicate that preparations for a seventh nuclear test are well underway. The think tank says it confirmed construction of new buildings and other activities immediately outside a new entrance to Tunnel No.3 located in the south of the site.

Pyongyang said that it shut down the Punggye-ri site four years ago.