A North Korean ship is docked in November 2017 at Rajin port, near North Korea's border city of Rason, adjacent to Russia and China

Russia and China on Tuesday blocked an American initiative that aimed to halt fuel deliveries to North Korea, which Washington accuses of exceeding its annual ceiling for 2019, diplomatic sources said.

Moscow and Beijing said more time was needed to study the US request, which was backed by 25 UN members including Japan, France and Germany, according to the sources.

A week ago, the United States, in a report, accused North Korea of breaching the United Nations-imposed ceiling on fuel imports by carrying out dozens of ship-to-ship transfers.

The cap on fuel imports is among a series of tough sanctions adopted by the UN Security Council in response to North Korea's ballistic missile and nuclear tests.

The United States insists that "maximum pressure" from the sanctions must remain on North Korea until it agrees to dismantle its weapons program.

Washington had asked that a United Nations sanctions committee rule that the annual cap of 500,000 barrels had been exceeded and order all countries to halt fuel deliveries.

Countries on the sanctions committee, including Russia and China, had until Tuesday to raise objections to the request to cut off fuel shipments to North Korea.

The issue "should be decided... on the basis of solid and credible facts and through abundant discussion and study," said Chinese foreign ministry spokesman Lu Kang at a press briefing in Beijing on Wednesday.

The relevant parties should "do more things that are conducive to easing the situation... and push forward the political settlement of the Korean Peninsula issue," he added.

The US and Japan have documented at least eight illegal ship-to-ship transfers of fuel involving North Korea-flagged tankers, according to the report obtained by AFP.

An additional 70 instances of fuel deliveries at sea were detected by the United States between January and April, although the volume of fuel unloaded from the ships was not known.

"The United States and its partners remain gravely concerned about the degree of UN Security Council resolution violations that are occurring in relation to North Korea's import of refined petroleum products," the report said.

According to diplomats, the North Koreans are masters at concealing ships via false flags or false documents to hide their use for the benefit of Pyongyang.

The United States last year accused North Korea of exceeding the quota on fuel imports through illegal ship transfers, but Russia and China raised questions about the data and no action was taken.

This year, Washington began raising its concerns about the sanctions-busting with Beijing and Moscow weeks ago, arguing that without enforcement, the cap on fuel imports will be "completely meaningless," a council diplomat said earlier.

Over the past year US President Donald Trump and North Korean leader Kim Jong Un have held two summits, but the second meeting, held in Vietnam, ended abruptly when the two failed to agree on what the North would be willing to give up in exchange for sanctions relief.

South Korean President Moon Jae-in called last week for Trump and Kim to meet again soon, saying a prolonged impasse could weaken their willingness to pursue dialogue on the Korean peninsula.