The United States said on Wednesday that it hopes to work with China to halt North Korea's nuclear program, but warned that if this fails it might resort to sanctions.
In an address to State Department staff, Secretary of State Rex Tillerson spoke of "third country sanctions," warning that Chinese companies may be hit for working with Pyongyang.
Tillerson said China has reaffirmed support for a "denuclearized" Korean peninsula but that Washington would test their commitment to help rein in their neighbor by "leaning into them hard".
He said Washington would urge world powers to fully implement UN Security Council sanctions against Kim Jong-Un's regime "because no one has ever fully implemented those."
And he warned: "We are going to to lean in to people to fully implement them. We tell them: 'We are watching what you are doing'.
"When we see you are not implementing, we see companies or we see individuals that are violating sanctions we are going to contact you, we are going to ask you to take care of it," he said.
"If you can't take care of or you simply don't want to care of it for own internal political reasons we will. We'll sanction them through third country sanctions."
China approved UN sanctions targeting North Korea's nuclear program, but American observers believe Chinese banks are continuing to give Pyongyang access to international finance.