The US Justice Department accused North Korea’s state-owned bank of evading US sanctions laws and charged 28 North Korean and five Chinese citizens in its largest crackdown on North Korea sanctions violations.

In a grand jury indictment made public on Thursday, US prosecutors accused North Korea’s Foreign Trade Bank (FTB) of conspiring with the employees charged to cause other banks “to process at least $2.5 billion in illegal payments via over 250 front companies.”

The United States blacklisted the FTB in 2013; the U.N. Security Council did so in 2017.

The indictment adds to mounting friction between Washington and Pyongyang after denuclearization talks launched by U.S. President Donald Trump and North Korean leader Kim Jong Un stalled.

The indictment is the largest of any North Korea sanctions violations case, a US law enforcement official said. The alleged crimes include money laundering and bank fraud.

Washington “has signified its commitment to hampering North Korea’s ability to illegally access the U.S. financial system and limit its ability to use proceeds from illicit actions to enhance its illegal WMD and ballistic missile programs,” Acting United States Attorney Michael Sherwin said in a statement.

Some of the $2.5 billion was directed to North Korea’s nuclear weapons and ballistic missile program, a US official said.

The transactions took place in China, Russia, Libya and Thailand, and many of those charged were bank employees, including two former presidents of the FTB and two former co-vice presidents.

North Korea has been subject to UN sanctions since 2006 that have been strengthened by the Security Council over the years in a bid to cut off funding for Pyongyang’s nuclear and ballistic missile programs.

North Korea continued to enhance those programs last year in breach of UN sanctions, according to a report this year to the U.N. Security Council.