Satellite imagery of North Korea’s main nuclear test site “strongly suggests” that preparations for the country’s sixth atomic test are entering the final stages, a leading U.S. website that monitors the isolated nation said Tuesday.

The 38 North website, a project of the U.S.-Korea Institute at Johns Hopkins University, said imagery taken Saturday of the Punggye-ri nuclear test site shows the continued presence of three to four vehicles and trailers at the entrance to an underground test tunnel.

According to the website, the texture of the ground from the tunnel entrance past the vehicles and trailers “suggests that communications cables may have been laid on the ground.” This equipment would likely be used to initiate the test and then collect and process data from the explosion. The report said water was also being pumped out of the portal or being drained downhill, “presumably to keep the tunnel dry for monitoring or communications equipment.”

“The combination of these factors strongly suggests that test preparations are well under way, including the installation of instrumentation,” 38 North said.

It also noted that there appeared to be no significant activity taking place at other areas of the test site.

“The lack of activity may mean that test preparations are in their final stages,” the report said.

It cautioned, however, that the imagery did not provide “definitive evidence” of an impending nuclear test.

“Since North Korea knows the world is watching and is capable of deception, caution should be used before declaring that a nuclear test is imminent,” the site warned.

The website had noted in an earlier report last week that the presence of vehicles at the test facility could indicate that instruments — and even a nuclear device — may have been installed for an underground blast.

Another report on the website, also posted Tuesday, said that a number of activities associated with the reprocessing of spent nuclear fuel rods were observed at the North’s Nyongbyon atomic research center. The activities could represent a plan to restart a reprocessing campaign to yield plutonium for nuclear weapons.

That drive was believed to have been completed in mid-2016, according to 38 North.

“The DPRK’s continued defiance of its UNSCR obligations only serve to increase the international community’s resolve to counter the DPRK’s prohibited weapons of mass destruction programs,” State Department spokesperson Katina Adams said in an email. The DPRK is the acronym for the North’s formal name, the Democratic People’s Republic of Korea, and UNSCR is a reference to United Nations Security Council resolutions.

“Our commitment to the defense of our allies, including the Republic of Korea and Japan, in the face of these threats, remains ironclad,” Adams added.

A sixth nuclear test by the North would come amid a flurry of activity by the country as it seeks to master the technology needed to mount a nuclear weapon on a long-range ballistic missile capable of striking the continental United States.

There has been growing speculation that Pyongyang will conduct an intercontinental ballistic missile test after leader Kim Jong Un used a New Year’s Day address to claim that the North was in the final stages of developing such a weapon.

Any nuclear or ICBM test would pose a fresh challenge to U.S. President Donald Trump, who has vowed that Pyongyang’s goal of possessing a nuclear-tipped long-range missile “won’t happen.”

U.S. Secretary of State Rex Tillerson said this month during a visit to Asia that years of efforts to get the North to give up its nuclear weapons program have failed. Tillerson promised “a new approach,” saying that military action against the North “was on the table.”

The international community has been piling pressure on the North after it conducted two nuclear tests and launched more than 20 missiles last year.

This month, Pyongyang also issued an unusually overt threat to Washington and Tokyo, firing off a simultaneous barrage of four missiles as part of exercises that it said were training for strikes on U.S. military bases in Japan.

Prime Minister Shinzo Abe characterized the exercise as exemplifying “a new level of threat” from the North.

Missile experts said the hypothetical target of that drill appeared to be U.S. Marine Corps Air Station Iwakuni in Yamaguchi Prefecture. Observers said the undisguised threat to U.S. bases in Japan was rare, even for Pyongyang, which routinely serves up colorful invectives.