Maritime Self-Defense Force ships have been deployed to waters around the Korean Peninsula, even near the Northern Limit Line, since late last year to thwart North Korean attempts to evade international sanctions, government sources say.

Following a request from the U.S. military in December, MSDF ships have been stationed in areas including the Yellow Sea to monitor whether refined oil is being transferred from foreign ships to North Korean vessels in violation of U.N. sanctions imposed on Pyongyang over its nuclear and missile programs, the sources said Friday.

The government may report on that effort Tuesday at an international meeting of foreign ministers regarding North Korean issues in Vancouver, British Columbia.

According to the sources, the MSDF is following the standard rules for warning and surveillance activities for the mission. The main purpose is to monitor ship movements and collect information to share with the United States. The MSDF does not forcibly inspect ships, as it is not authorized to do so unless certain conditions are met under the Self-Defense Forces Law.

But the government believes the activities will put more pressure not only on North Korea, but also on China and Russia, which are allegedly economic enablers of North Korea’s weapons development programs.

So far, the MSDF has sent vessels to areas where suspicious ships are found by its P-3C surveillance planes during daily patrols over the East China Sea and elsewhere.

In the Yellow Sea, to the west of the Korean Peninsula, the MSDF has sometimes sailed as far as the Northern Limit Line, the de facto maritime border between North and South Korea, the sources said.

The MSDF takes photos of suspicious ships and provides information to the U.S., which is leading global efforts to impose tough economic sanctions on North Korea, according to the sources.

Missions also take place in the Sea of Japan.

A senior MSDF official, however, admitted that it is not clear whether the efforts will yield substantial results in foiling oil smuggling.

“We cannot forcibly investigate ships. They could leave the area and sail to a different location to meet (a North Korean ship) and transfer the items,” the official said. “The activities have significance in tightening the net around North Korea, but the actual effects remain uncertain.”

The U.N. Security Council has imposed stepped-up sanctions against North Korea 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 or from North Korean-flagged vessels at sea.

Following Pyongyang’s test-firing in late November of a new 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 products exported to North Korea.

But media reports said a Hong Kong cargo ship and Russian tankers had transferred refined oil to North Korean ships at sea despite the ban. South Korean and other media have also reported the possible involvement of Chinese ships in suspected North Korean smuggling activities.