BEIJING -- China is informally telling some domestic businesses to stop hiring North Koreans, a diplomatic source said on Saturday.

The whisper campaign is apparently a form of economic sanction as North Korea plows ahead with its ballistic missile development program.

The United Nations in 2015 estimated that there were 50,000 North Koreans laboring overseas and sending up to $2.3 billion worth of foreign currency back home.

Of these workers, tens of thousands are thought to be in China.

According to a source who is familiar with China-North Korea diplomacy, Beijing began instructing Chinese businesses to refrain from hiring North Korean nationals in March 2016 -- the month that the U.N. toughened sanctions on the country in response to Pyongyang's fourth nuclear test.

The instruction has so far been given informally, and in some cases, orally. No formal notices have been issued, the source said.

The companies receiving the instruction are mainly in Jilin and Liaoning provinces, on the border with North Korea. Beijing appears to be gradually including more companies in its whisper campaign, the source said.

The informal sanction appears to contradict the Chinese foreign ministry's position that the country should not impose any form of sanction against North Korea if it is not based on a U.N. Security Council resolution. At the same time, it is a means by which Beijing can register its displeasure with Pyongyang's missile and nuclear testing.

The U.N. estimations mentioned earlier are just that. Since many of these workers enter destination countries illegally, it is difficult to gauge just how many there are and how much they might earn.

A South Korean media outlet estimates that more than 100,000 North Koreans work abroad.