BEIJING (Reuters) - China has realized it has limited time to rein in North Korea’s nuclear program through negotiations and is open to further sanctions against Pyongyang, a senior U.S. State Department official said on Friday.

FILE PHOTO: Chinese dredging vessels are purportedly seen in the waters around Mischief Reef in the disputed Spratly Islands in the South China Sea in this still image from video taken by a P-8A Poseidon surveillance aircraft provided by the United States Navy May 21, 2015. U.S. Navy/Handout via Reuters/Files

Susan Thornton, the acting assistant secretary for east Asian and Pacific affairs told a news briefing in Beijing that China understood that the U.S. viewed the North Korea situation as an urgent “time-limited problem set”.

“So they know now that they don’t have, I think, as much time to try to bring the North Koreans to the table to get their calculus changed and get them to the negotiating table,” she said. “And I think that has lent some urgency to their measures.”

Pyongyang has conducted dozens of missile tests, the most recent last Sunday, and tested two nuclear bombs since the start of 2016, in defiance of U.N. Security Council resolutions. It says the program is necessary to counter U.S. aggression.

Thornton said the United States was looking at discussing with China a new U.N. Security Council resolution on pre-negotiated measures to reduce delays in any response to further nuclear tests or other provocations from the North.

While China believes sanctions against North Korea “don’t work overnight”, Thornton said there were no indications Beijing had gone cold on potentially implementing more of them against Pyongyang.

“Their calculus about how much pressure to impose on North Korea is related to their tolerance for potential instability, which is low, I would say,” she said.

Asked about Thornton’s comments, Foreign Ministry spokesman Lu Kang said China supports the denuclearisation of the Korean peninsula through talks and strictly implements U.N. Security Council resolutions.

“We always maintain that all relevant parties should display flexibility and move forward together to return to the negotiating table as soon as possible,” Lu told a daily news briefing.

Foreign Minister Wang Yi said on Wednesday no one had the right to bring chaos to the Korean peninsula, a day after China pushed for full implementation of U.N. sanctions and called for dialogue.

The North has proudly publicized its plans to develop a missile capable of striking the United States and has ignored calls to halt its weapons programs, even from ally China.

Even as Washington seeks greater Chinese cooperation on North Korea, a U.S navy warship sailed within 12 nautical miles of Mischief Reef in the disputed Spratly Islands in the first such maneuver since President Donald Trump took office.

It prompted an angry response from Beijing, which accused the United States of jeopardizing a recent easing of tension between China and other claimants in the region, particularly the Philippines.

Thornton said Washington’s policy on the South China Sea had not changed under President Donald Trump, and it remained supportive of diplomatic processes.

“It doesn’t mean we are going to change our military presence or our security commitments in the region,” she said. “Those need to stay and they need to be there, and that will give confidence to the diplomatic process, we think.”

She said freedom of navigation operations were not the central part of U.S. policy in the South China Sea.

“But to the extent that we continue to do it, we don’t shy away from doing it, that’s a continuation,” she said.