Rex Tillerson
Rex TillersonReuters

U.S. Secretary of State Rex Tillerson on Wednesday urged Chinese officials to apply greater diplomatic and economic pressure on North Korea to force Pyongyang to rein in its nuclear weapons program, AFP reports.

Tillerson's remarks came after he and Defense Secretary chief Jim Mattis met with the Chinese visitors at the State Department, where Mattis said he saw scope for an improved defense relationship.

The Trump administration's policy hinges on pressuring North Korea through China, but the President on Tuesday offered a grim assessment of China's role in restraining North Korea, writing on Twitter, "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!"

On Wednesday, calling North Korea the "top security threat" to the United States, Tillerson said China has a "diplomatic responsibility to exert much greater economic and diplomatic pressure on the regime if they want to prevent further escalation in the region."

Tillerson said the U.S. and Chinese officials had agreed that companies from both countries should not do business with any UN-designated North Korean entities.

North Korea has continuously carried out nuclear tests and ballistic missile tests in violation of UN resolutions.

The West is concerned that North Korea may be on a path to developing an intercontinental ballistic missile (ICBM).

On Tuesday, it was reported that U.S. spy satellites had detected new activity at North Korea's underground nuclear test site.

The activity appears to involve some modifications around one of the tunnel entrances to an underground test area, officials said.