WASHINGTON (Reuters) - U.S. officials can open personal mail arriving from abroad as part of the fight against terrorism, and do so when they deem it necessary to protect the country, a Customs and Border Protection spokeswoman said on Monday.

News of the little-known practise follows revelations that the government approved eavesdropping on U.S. citizens without judicial oversight after the September 11 attacks, which sparked concern from civil liberties advocates and some lawmakers who called for congressional hearings.

"Customs and Border Protection is charged with making sure that terrorists and terrorists' weapons don't enter the country," said Suzanne Trevino, a spokeswoman for the customs agency, which is part of the Department of Homeland Security.

"One of our areas of responsibility is to inspect international mail coming into our country," she said. "We respect privacy and always keep that at the forefront, but at the same time we need to make sure we do our job in keeping U.S. citizens safe."

Customs and Border Protection's web site notes that "all mail originating outside United States Customs territory that is to be delivered inside U.S. Customs territory is subject to Customs examination."

Grant Goodman, an 81-year-old retired University of Kansas history professor, drew attention to the policy after a letter he received from a colleague in the Philippines was opened and resealed by Customs and Border Protection, and only then sent on to him.

He said he was shocked and amazed that the letter -- which he received last month from another retired history professor with whom he has corresponded for 50 years -- had been screened.

"It was a big surprise," Goodman told Reuters. "The public should know that this is being done. Nobody whom I know had any idea that this was going on. And as far as I know, it's never been announced. It's never been revealed that this is being done."

Trevino said she could not comment specifically on Goodman's case because she had not seen the letter and was not sure if it had been examined as he said. She did not say how often officials open mail for security reasons.