ZURICH, Nov 17 (Reuters) - The head of the U.S. Justice Department's tax division has warned that Swiss banks which do not come forward under a government-brokered programme risk prosecution over tax evasion by their U.S. customers.

The programme closes in about six weeks.

"We want to assure the banks that we are here to speak with them and implement the programme," Kathryn Keneally said in a interview with Swiss newspaper Sonntagszeitung on Sunday.

At the same time, Keneally said the Justice Department continues to investigate banks which aided tax evasion with information from banks already cooperating with U.S. officials and will seek criminal charges for those which don't come forward.

"Our program enables banks and individuals to bring this to an end. Should they choose not to make use of it, we will keep investigating and press charges," Keneally said.

It applies to about 100 second-tier Swiss banks, which could have to disclose some previously hidden information and face penalties of up to 50 percent of the value of assets they managed on behalf of wealthy Americans.

It does not cover banks already under U.S. criminal investigation, which include some of Switzerland's biggest banks such as Credit Suisse and Julius Baer.

Keneally said Justice Department officials have received various requests from Swiss banks to interpret the program's guidelines, but was hesitant to enter talks unless they centered on a specific institute's problems.

She didn't comment on the investigation into Credit Suisse and other banks being targeted, or on potential fines.

Swiss bankers which cooperate individually with U.S. Justice Department officials run the risk of being pursued by Swiss prosecutors for violating the country's strict banking secrecy laws.