NEW YORK, Jan 10 (Reuters) - Moody’s Investors Service said on Thursday the United States’ “triple-A” government bond rating could come under pressure in the very long-term if the Medicare and Social Security programs are not reformed.

“These two programs are the largest threats to the long-term financial health of the United States and to the government’s Aaa rating,” Moody’s analyst Steven Hess said in the agency’s annual report on the United States.

The report is not a rating action.

Hess also said that risks from the U.S. subprime mortgage crisis are not affecting the nation’s credit rating.

However, the housing downturn and subprime crisis could result in “a period of slower growth in coming quarters, although further interest rate cuts by the Federal Reserve could help to maintain positive growth,” he said.

John McCarthy, director of foreign exchange at ING Capital Markets in New York, noted that “some are saying comments about the possible downgrade of the U.S. long-term rating is hitting the dollar a bit.”

The United States’ “Aaa” foreign currency ceiling and “Aaa” bond ratings and stable outlook are supported by the nation’s large and diverse economy and moderate level of debt, Moody’s said.

Federal debt ratios relative to gross domestic product and to revenue appear “set to improve modestly in the next few years,” the agency said. (Reporting by Neil Shah; additional reporting by Steven Johnson; Editing by Dan Grebler)