Shortly after the inter-Korean summit last month, the U.S. Treasury Department telephoned every big South Korean bank and told them to abide by sanctions against North Korea, it emerged in a National Assembly audit on Friday. The U.S. asked seven commercial and state-run banks about their North Korea-related business plans and cautioned them not to go too far. They could of course become targets of U.S. sanctions if they engage in any dealings with North Korea.

In the audit, the South Korean ambassador to the U.S. admitted that he had received a complaint from Washington that Seoul was "moving too quickly" in rapprochement with the North, which cannot be forced to give up its nuclear weapons if sanctions are undermined.

Earlier, a State Department official warned the heads of South Korean companies who look forward to doing business with North Korea not to view exceptional authorizations in dealing with the North as an easing of overall sanctions. Foreign Minister Kang Kyung-wha invited U.S. President Donald Trump's displeasure when she suggested that South Korea could ease some of its own unilateral sanctions against the North. "Well they won't do it without our approval," Trump blurted out. "They do nothing without our approval."

These incidents clearly show that the U.S. now regards South as the weakest link among its allies in international sanctions against North Korea. And it is not the only country that is being given that impression. France's Le Monde newspaper quoted Moon Chung-in, a special security adviser to President Moon Jae-in, as saying that the South is "officially considering easing sanctions against North Korea" and that Moon is visiting France to "support North Korea." South Korea's finance minister, meanwhile, met with the heads of the International Monetary Fund and World Bank and asked them to "play an active role" in supporting North Korea's reforms and efforts to join the international community. No wonder the U.S. is alarmed.

Since early this year, Seoul has applied for a welter of sanctions exemptions -- to engage in exchanges of people and provide fuel and materials to North Korea during the Winter Olympics in Pyeongchang, for reunions of families separated by the Korean War, and for a cross-border military hotline. The U.S. granted them but is now rightly alarmed at their frequency in the absence of any progress in North Korean denuclearization.

Washington, by contrast, is steadfastly maintaining sanctions against North Korea as negotiations continue because otherwise there would be no leverage to get the North to abide by its pledge to scrap its nuclear weapons. Even Trump, who has praised North Korean leader Kim Jong-un to the skies and joked that the two have fallen "in love," said he could not ease sanctions without a quid-pro-quo from the North. That is what Moon should be saying too, and that he is not saying it could spell disaster.

