Just before the deadline Russia, supported by China, put a “hold” on the complaint, which delays any action for six months, the diplomats said.

The United States had no immediate comment on the move by Russia and China, which appeared to undermine what the Americans have previously described as a strong commitment by the Security Council to enforce the North Korean sanctions.

But questions about the willingness of other countries — particularly China and Russia — to maintain the sanctions pressure on North Korea have grown since President Trump met with the North Korean leader, Kim Jong-un, last month in Singapore.

Mr. Trump has described that meeting in glowing terms and suggested the North Korean nuclear threat has eased, even though Mr. Kim has made no concrete commitments yet to denuclearize, raising doubts about his intentions.

If the North Koreans are successfully subverting the sanctions on imported petroleum, Mr. Kim would be under less pressure to disarm.