North Korea is continuing to mass resources at a known weapons testing site inside the country, a defense source told Fox News on Wednesday, prompting worries Pyongyang could be plotting to greenlight another provocative nuclear bomb test amid heightened tensions following the death this week of an American student who had been imprisoned by Kim Jong Un’s rogue regime.

“There are more people and more cars but nobody knows what that means,” a defense source said about surveillance of the nuke test site. The source added the buildup had been occurring for months.

North Korea is relentlessly pursuing its goal of building a nuclear bomb that can be mounted on an intercontinental ballistic missile. Pyongyang has already conducted five nuclear tests and recently launched an advanced missile that suggests a functioning ICBM may be within reach.

Seeking to deter North Korea, the Trump administration has so-far leaned on China, one of the few countries able to exert significant economic influence over Pyongyang. China told North Korea not to conduct a sixth nuclear test or risk sanctions, Secretary of State Rex Tillerson told “Special Report” in April.

“We’re asking a lot of the Chinese,” Tillerson said. “We are going to test China’s willingness to help address this serious threat.”

But any hope of de-escalating the North Korea menace was dealt a serious blow on Monday when Otto Warmbier died.

Warmbier, a college student who had been part of a tour group visiting North Korea, was detained by the regime for 17 months, during which he was subject to a show trial and unknown cruelties before he was suddenly returned to the U.S. last week – in a comatose state. Doctors weren’t able to find evidence of fractures, meaning it was unlikely Warmbier was severely beaten; however, the cause of his coma couldn’t immediately be deduced and President Trump said Warmbier suffered “awful torturous mistreatment…at the hands of the North Koreans.” North Korea claims Warmbier slipped into the vegetative state after contracting botulism and taking a sleeping pill.

Trump appeared significantly moved by Warmbier’s death, tweeting Tuesday that “the U.S. once again condemns the brutality of the North Korean regime as we mourn its latest victim” before signaling that China’s chance to resolve the crisis via diplomatic means may have come and gone.

Even as intelligence indicated North Korea was continuing to move materials to the nuke test site, Trump tweeted: “While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out. At least I know China tried!”

Earlier, Trump cryptically said the U.S. would “be able to handle it.”

North Korea will likely be a major topic when U.S. and Chinese diplomats meet in Washington, D.C. on Wednesday for the first Diplomatic and Security Dialogue, a summit that arose from the meeting between Trump and Xi in April. Tillerson and Secretary of Defense James Mattis are set to represent the U.S.