Commercial satellite images suggest a resumption of construction activity at North Korea's nuclear testing ground nearly four years after leader Kim Jong-un declared the site's closure and invited foreign journalists to observe the destruction of tunnels.

Analysts say it is unclear how long it would take North Korea to restore the site for nuclear detonations if it intends to do so. The site in Punggye-ri in the country's north east was used for its sixth and last nuclear test in 2017.

The sighting of construction activity at the site comes amid a deepening diplomatic freeze since the collapse of the second meeting between Mr Kim and then US president Donald Trump in February 2019, when the Americans rejected North Korean demands for major sanctions relief in exchange for a partial surrender of its nuclear capabilities.

Pyongyang has used the pause in talks to further expand its military capabilities, including nine rounds of missile launches so far this year. The unusually fast pace indicates an intent to pressure the Biden administration, which has offered open-ended talks but shown no willingness to concede on sanctions.

Mr Kim presided over a ruling Workers' Party meeting in January where Politburo members denounced what they described as US hostility and issued a veiled threat to resume tests of nuclear explosives and long-range missiles that the leader unilaterally suspended in 2018 to create diplomatic space with Mr Trump.

Some experts say Mr Kim is reviving an old playbook of brinkmanship to extract concessions from Washington and his neighbours as he grapples with a decaying economy crippled by the pandemic, mismanagement and US-led sanctions.

The new construction at Punggye-ri was first reported in an analysis of satellite images from Maxar Technologies by Jeffrey Lewis and Dave Schmerler, at the James Martin Centre for Nonproliferation Studies at the Middlebury Institute of International Studies.

Satellite photos from Planet Labs PBC analysed by the Associated Press showed two new structures appeared to have been built to the south of the site between February and early March. Those images were taken on March 6, two days after the Maxar images used by Mr Lewis and Mr Schmerler.

The analysts said the construction work was the first activity seen at the site since May 2018, when North Korea invited a group of foreign journalists to observe the destruction of tunnels at the site. North Korea did not invite outside experts capable of certifying what had been destroyed.

"We see very early signs of activity at the new site, including construction of a new building, repair of another building, and what is possibly some lumber and sawdust," Mr Lewis and Mr Schmerler wrote.

"North Korea uses a substantial amount of wood at the site both for buildings and shoring up tunnels. These changes have occurred only in the past few days."

This indicates North Korea has made a decision about the status of the site and possibly plans to bring it back to a state of readiness to resume nuclear testing, the analysts said.

"The test site is many months, if not years, from being ready for North Korea to conduct nuclear explosions there," they wrote.

"How long it would take North Korea to resume explosive testing at the site depends on the extent of the damage to the tunnels themselves, something we do not know with confidence. It is also possible that North Korea will resume nuclear testing at another location."

Some South Korean analysts have said the North could resume nuclear tests in the coming months to pressure the Biden administration, which has been preoccupied with Russia's invasion of Ukraine and a regional confrontation with China.

The US intelligence community's annual threat assessment, published on Tuesday, also warned there could be a new nuclear test by North Korea this year.

"In January, North Korea began laying the groundwork for an increase in tensions that could include (intercontinental ballistic missile) or possibly a nuclear test this year - actions that Pyongyang has not taken since 2017," the assessment said.

The weapons North Korea flight-tested this year include a purported hypersonic missile designed to evade regional missile defence systems. Some experts say Pyongyang may use another nuclear test to claim it has acquired an ability to produce a nuclear warhead small enough to fit on that missile.

Following North Korea's latest launch on Saturday, which it claimed was related to technologies for a spy satellite it is developing, South Korea's presidential office revealed that Seoul is closely monitoring the Punggye-ri site along with other North Korean nuclear and missile facilities.