The Russian Foreign Ministry on Saturday denied claims that U.N. sanctions against North Korea had been breached by Russian tankers transferring fuel to North Korean tankers at sea.

The statement came in response to a Reuters report citing unidentified Western European security sources who said the transfers took place in October and November and represented a breach of sanctions.

The Russian Foreign Ministry said in a statement carried by Russian news agencies that Russia has "fully and strictly observed the sanctions regime."

The statement did not address whether or not Russian ships had transferred the fuel, according to AP. It did say resolutions by the United Nations Security Council have imposed limits on North Korea's refined oil imports but haven't banned them altogether.

The council has unanimously approved several rounds of sanctions against North Korea over its missile tests and nuclear program, including a tough new U.N. resolution earlier this month. The resolution adopted by the council included sharply lower limits on North Korea's refined oil imports, the return home of all North Koreans working overseas within 24 months, and a crackdown on ships smuggling banned items including coal and oil to and from the country. It didn't include even harsher measures sought by Washington that would ban all oil imports and freeze international assets of the government and its leader, Kim Jong Un.

The U.S. State Department, in a statement, called on Russia and other U.N. members to "strictly implement" sanctions on North Korea and to work "more closely together to shut down U.N.-prohibited activities, including ship-to-ship transfers of refined petroleum and the transport of coal from North Korea."

North Korea relies on imported fuel to keep its struggling economy functioning. It also requires oil for its intercontinental ballistic missile and nuclear program that the United States says threatens the peace in Asia.

China on Friday denied reports it had been illicitly selling oil products to North Korea in violation of U.N. sanctions, after U.S. President Donald Trump said he was unhappy that China had allowed oil to reach the isolated nation.

China's denial came a day after it blocked a U.S. effort at the United Nations to blacklist six ships Washington believes had engaged in illicit trade with North Korea, a U.N. Security Council diplomat said.

According to documents seen by Reuters this month, the United States had proposed that the U.N. Security Council blacklist 10 ships for illicit trade with North Korea. It accused the vessels of "conducting illegal ship-to-ship transfers of refined petroleum products to North Korean vessels or illegally transporting North Korean coal to other countries for exports." Three North Korean ships among the 10 were blacklisted, along with a Panama-registered vessel.