A Japanese patrol plane spotted contact between tankers of North Korea and Commonwealth of Dominica in the East China Sea, raising suspicion of a possible transfer of cargo in violation of U.N. sanctions on the North, government sources have said.

The Maritime Self-Defense Force P-3C aircraft took photos of the scene near Shanghai on Saturday as it was patrolling for illicit transfer of refined oil at sea by North Korea.

Foreign Minister Taro Kono said Tuesday that the government will take tougher measures on such activity by North Korea.

“North Korea is becoming more skilled at escaping sanctions,” Kono told a news conference. “We will take a coordinated response to it internationally.”

The information was passed on to the U.S. government, Japan’s key ally in global efforts to impose tough economic sanctions on North Korea, the sources said.

The U.N. Security Council has imposed stepped-up sanctions against the North as it continues to test-fire missiles and carry out nuclear tests.

Last September, a council resolution prohibited the ships of U.N. member states from engaging in the transfer of any goods or items to North Korean-flagged vessels at sea.

Following Pyongyang’s test-firing in late November of a new type of intercontinental ballistic missile it claims is capable of striking anywhere in the United States, a new sanctions resolution was adopted, targeting around 90 percent of refined petroleum product exports to North Korea.

At the request of the United States, MSDF vessels and patrol aircraft have engaged in patrolling the sea near Japan to look for possible cases of oil smuggling from late last year.

Defense Minister Itsunori Onodera refrained from commenting on details of the MSDF activities but told reporters Monday, “We will ensure the effectiveness of U.N. Security Council resolutions by working closely with the international community, not only the United States and South Korea, but also China and Russia.”

China and Russia are believed to be economic enablers of North Korea’s nuclear weapons and ballistic missile development programs.