The U.S. is stepping up sanctions against North Korea ahead of Secretary of State Mike Pompeo's fourth visit to the North late this month.

After U.S. President Donald Trump's initial generosity toward North Korean leader Kim Jong-un, Washington has gradually returned to a policy of maximum pressure against the North while Trump's attentions are otherwise engaged.

The U.S. Commerce Department on Tuesday blacklisted six Russian ships and the two companies that own them for illegally transporting fuel to North Korea. This is the third unilateral sanction against North Korea the U.S. has announced this month.

One of the Russian tankers, the Patriot, violated sanctions by loading fuel on two North Korea ships early this year which are on the U.S. sanctions list. Washington believes the oil was purchased by Room 39, which handles Kim's private coffers.

Senator Ed Markey, a ranking member of the Senate Foreign Relations Subcommittee on East Asia, in a statement that Russian companies seeking to aid the Kim Jong-un regime deserve "extensive intervention" and urged more efforts to prevent the North from obtaining foreign currency.

The Senate also raised the bar by setting denuclearization as the "minimum requirement" for any declaration ending the Korean War. Senator Jack Reed, a ranking member of the Armed Services Committee, told Voice of America that there are "many conditions" that need to be satisfied before the end of the Korean War can be declared, and specifically cited the reporting of North Korea's nuclear facilities and speedy denuclearization. David Purdue, another member of the committee, said the North's denuclearization is the "top priority" and peace negotiations come next.

U.S. officials are also worried that warming ties between North and South Korea may prove a weak link in pressuring the North. Cheong Wa Dae has downplayed the concerns, saying any steps it takes toward improving ties with the North are coordinated with the U.S.

President Moon Jae-in's special security adviser Moon Chung-in said despite the concerns the Korean government is to keep up its engagement policy with North Korea. "U.S.-North Korea talks are stalled, and... Seoul aims to prod both sides to keep moving in the direction of disarmament and peace," he told the Wall Street Journal on Wednesday.