The authorities in a northeastern Chinese border city have secretly warned local firms they will be slapped with fines if they newly hire North Korean workers in violation of U.N. sanctions, amounting to 5,000 yuan ($750) per worker, sources familiar with the situation said Wednesday.

The move by the city government of Dandong indicates that Beijing has quietly ramped up its pressure on Pyongyang in line with tougher sanctions in response to North Korea's weapons development, as it paves the way for U.S. President Donald Trump's planned visit to China in November.

Dandong authorities have not made public a series of notices given to local businesses, including the fines' imposition plan, as China wants to avoid provoking the reclusive neighboring country, the sources said.

Based on the U.N. Security Council resolution adopted on Aug. 5 banning U.N. members from increasing the number of North Korean laborers working abroad, the city has notified the firms that it will impose a 5,000 yuan penalty for each new worker hired on Aug. 5 or later, and will deport them back to North Korea, according to the sources.

Companies are asked to voluntarily send back North Korean workers hired on or after that date, and their failure to do so will be subject to additional penalty charges, the sources said.

The August sanctions were adopted in the wake of North Korea's two long-range missile tests in July.

Similarly targeting North Korea's sources of foreign currency, the Security Council approved another sanctions -- banning North Korea from exporting textiles -- on Sept. 11, following the country's sixth and largest nuclear test on Sept. 3.

Based on the step, the Chinese city authorities made clear that textiles made in North Korea using materials imported from China will also be covered by a trade embargo. The Chinese firms have been banned from purchasing such textile products effective Sept. 11, according to the sources.

Roughly 70 percent of all trade between China and North Korea is said to pass through the border city in Liaoning Province.

The measures taken by the city government will "have a big impact because we had been able to hire North Korean workers on the cheap," said a senior official of a factory in Dandong that currently employs North Korean workers.

In the Sept. 11 sanctions, U.N. members are called on not to grant work permits for North Korean migrant workers.

While not issuing new work permits, the Dandong city authorities have allowed those who already had relevant permits to remain in China until their permits expire.

The moves in the border city came to light following steps taken by four major Chinese banks in September to freeze accounts held by North Koreans.