Washington seems to have been alarmed by recent suspicions that the South Korean government dragged its heels over illegal shipments of North Korean coal in its ports.

Another sanctions advisory about maritime shipments in February was only published with a Chinese translation. That may suggest that Washington now considers South Korea a liability alongside China and Russia, which have already called for easing sanctions against the rogue regime.

The U.S. State Department has pointedly published a Korean translation of an advisory against violating North Korea sanctions, in a move that sensitive observers interpret as a shot across the bow of the Korean government. The advisory on the State Department's website has also been translated into Chinese, Russian, French, and Spanish.

KOEN, a subsidiary of power monopoly KEPCO, is being investigated by the Korea Customs Service in connection with some 9,000 tons of North Korean coal disguised as Russian product that arrived in Incheon and Pohang last October.

It has now emerged that since November, three other cargo vessels shipped another 15,000 tons of suspected North Korean coal from a Russian port to South Korea. "If it turns out that the coal was from North Korea, KEPCO could be subject to sanctions," a diplomatic source said. "Washington worries that Seoul could break ranks with the international sanctions regime."

Foreign Minister Kang Kyung-wha, national security adviser Chung Eui-yong, and National Intelligence Service chief Suh Hoon have all made trips to Washington in recent weeks to allay these fears.

Pyongyang has kept up its own campaign against the sanctions. "Contrary to the joint statement of the U.S.-[North] Korea summit in Singapore, the U.S. is even talking about 'controlling the speed' of inter-Korean relations, while making unilateral demand for denuclearization and insisting on 'maximum pressure,'" the official Rodong Sinmun complained Thursday.

The State Department advisory warns against buying North Korean products and using North Korean labor, saying, "Multiple U.S. and UN sanctions impose restrictions on trade with North Korea and the use of North Korean labor, potentially impacting a company's supply chain operations."

"Businesses should be aware of these deceptive practices in order to implement effective due diligence policies, procedures, and internal controls," it adds.

The deceptive practices include fake country-of-origin labels or goods whose prices are much lower than market prices. It also advises nations to find out if North Korean laborers are involved in the production of certain products, while listing 42 countries that employ slave labor from the North, including China and Russia.

It warns that foreign banks could be banned from dollar-based trading, saying that their proxy bank accounts in the U.S. will be confiscated, if they are involved in intentional transactions with the North.

U.S. lawmakers are calling for far stronger sanctions against the North despite Pyongyang's recent return of the remains of American soldiers who died in the Korean War. They unanimously opposed the idea of reopening the joint Korean Kaesong Industrial Complex, which South Korea's Unification Ministry hinted at on Wednesday.

Sen. Cory Gardner, the chairman of a Senate Foreign Relations Subcommittee, told Voice of America that reopening the industrial park would be "a big mistake." He said any South Korean importers of North Korean coal should be punished.

Underlying the push is a flurry of reports that North Korea continues to develop nuclear weapons and missiles in secret. U.S. Secretary of State Mike Pompeo said in a congressional hearing last month that the North continues to produce nuclear materials.

The UN, in a report released Tuesday, said that the most important issue pending at the UNSC is not easing North Korea sanctions but implementing them properly, Radio Free Asia reported.