WASHINGTON (Reuters) - The United States has filed sealed criminal charges for the attack in Benghazi, Libya, last year in which the U.S. ambassador to Libya and three other Americans were killed, media reports said on Tuesday.

The U.S. Justice Department said it had no comment on the reports by CNN and the Wall Street Journal, but that its investigation was ongoing and remained a top priority.

It was not clear how many people had been charged and what the charges were.

Republican Representative Darrell Issa, an outspoken critic of President Barack Obama's administration, said in a statement that it is "critical" that anyone being charged "be questioned and placed in custody of U.S. officials without delay."

Obama's administration has been under domestic pressure to find those responsible for the September 11, 2012, attack on a U.S. diplomatic and intelligence outpost.

Under questioning from lawmakers in May, U.S. Attorney General Eric Holder said investigators had made "very, very, very substantial progress."

"We are at a point where we have taken steps that I would say are definitive, concrete, and we are - we will be prepared shortly I think to reveal all that we have done," said Holder, the chief U.S. law enforcement official.

CNN reported that charges were filed against Ahmed Abu Khattala, a Libyan militia commander. U.S. government sources had previously described Abu Khattala as a suspect.

The charges were under seal in New York and were the first criminal counts to emerge, CNN said, citing unidentified people briefed on the investigation.

U.S. officials are prohibited by law from discussing matters that are under seal in a court. An FBI spokeswoman declined to comment.

Citing unidentified people familiar with the matter, the Wall Street Journal reported that there were charges against "a number of suspects" in the attack.

(Reporting by David Ingram; Editing by Howard Goller, Eric Beech, Mohammad Zargham and Lisa Shumaker)