By Idrees Ali

WASHINGTON (Reuters) - About 11,000 U.S. troops are serving in Afghanistan, the Pentagon said on Wednesday, thousands more than it has previously stated.

The announcement by Pentagon officials at a news conference did not represent an increase in troops in Afghanistan and came after U.S. Defense Secretary Jim Mattis expressed frustration with the method of counting U.S. troops in conflict zones.

The Pentagon said previously that there were roughly 8,400 U.S. troops in Afghanistan, under a cap set during President Barack Obama's administration.

While the Pentagon said the move was a step in increasing transparency, it did not provide counts of U.S. troops in Iraq and Syria.

"This is not an attempt to bring more forces in, but it is an attempt to actually clarify a very confusing set of reporting rules that has the unintended consequence of forcing commanders to make readiness trade-offs," said Lieutenant General Kenneth McKenzie, joint staff director.

"So what it does is, it actually lets the American people know what their sons and daughters are doing in Afghanistan," McKenzie said.

Previously disclosed troop numbers did not reflect the extent of the U.S. commitment on the ground since commanders sometimes brought in forces temporarily to get around the Obama-era limits.

Mattis has said that he would wait for a complete count of U.S. troops in Afghanistan before making a decision on how many additional troops to send, calling the accounting system strange.

Mac Thornberry, the Republican chairman of the U.S. House Armed Services Committee, welcomed the change.

"The Obama Administration did not shoot straight on how many people they sent to Afghanistan, which added cost to the mission and made it harder to succeed," Thornberry said.

On Iraq and Syria, Pentagon spokeswoman Dana White said accounting for troop numbers was still under review.

The force levels, which are officially at 5,262 in Iraq and 503 in Syria, are believed by U.S. officials to be more than a couple of thousands troops shy of the actual number of U.S. forces in both countries.

"We are reviewing Iraq and Syria and the same guiding principals will govern how we roll out those numbers as well," White said.

U.S. officials have suggested that the delicate political situation in Iraq could be one reason why the United States is cautious in releasing a full picture of forces in the country.

(Reporting by Idrees Ali and Warren Strobel; editing by Cynthia Osterman and Grant McCool)