The United States and China traded barbs at a U.N. drugs meeting on Monday, with Washington accusing Beijing of failing to stop sales of precursor chemicals for fentanyl and China dismissing the allegation as false while calling the U.S. irresponsible.

The exchange, delivered in separate statements at the U.N.'s annual Commission on Narcotic Drugs meeting in Vienna, underscored tensions between the two countries over illicit drugs and tariffs, with their leaders due to meet in China at the end of the month.

"We know where the chemical precursors (for fentanyl) are coming from. They are manufactured by the millions of tons in China," Sara Carter, director of the White House Office of National Drug Control Policy, said as she delivered the U.S. statement.

"We know that China's weak export controls and lax enforcement allow its chemical industry to foster friendships with the (drug) cartels. At the same time, China's overly effective controls over rare earth minerals wreak havoc on legitimate industries."

Under an agreement struck in South Korea last year between President Donald Trump and his Chinese counterpart Xi Jinping, the U.S. agreed to trim tariffs on China in exchange for Beijing cracking down on the illicit fentanyl trade, resuming U.S. soybean purchases and keeping rare earths exports flowing.

The U.S. Supreme Court last month invalidated a 10% fentanyl-related tariff Trump had imposed on China and others under an emergency statute. The Trump administration has told Beijing it expects to reimpose that levy under a different law, a U.S. official said.

"A certain country using the drug problem as a pretext has resorted to unilateral bullying and even interfered in the internal affairs of other countries, which ... gravely harms global cooperation in drug control," China's statement delivered by envoy Gao Wei said, apparently referring to the United States.

"It is regrettable that just now the U.S. delegate again made remarks that do not reflect reality," he said.

Countries should address domestic drug problems by improving control measures and engaging in international cooperation, not by "abusing sanctions, tariffs, or other means to erect barriers (and) shift blame," he added.