By Andrew Gray

WASHINGTON, April 21 (Reuters) - A drop in U.S. troop levels in Iraq is "inevitable over time" but U.S. forces face many more years of combat against "violent jihadist networks" around the world, Defense Secretary Robert Gates said on Monday.

Gates also said a war with Iran would be "disastrous on a number of levels" but Washington must not exclude the possibility of military action against Tehran.

The Pentagon chief made the remarks in a speech to cadets at the U.S. Military Academy at West Point, New York, according to a text released by the Pentagon ahead of the evening event.

While stressing the United States must remain engaged in Iraq "to avoid an even uglier fight or even greater danger to our country in the future," Gates said troop levels would decline further.

"A drawdown of U.S. force levels in Iraq is inevitable over time -- the debate you hear in Washington is largely about pacing," Gates said.

"But the kind of enemy we face today -- violent jihadist networks -- will not allow us to remain at peace," he said.

"What has been called the 'Long War' is likely to be many years of persistent, engaged combat all around the world in differing degrees of size and intensity," Gates said.

"This generational challenge cannot be wished away or put on a timetable. There are no exit strategies."

The United States has 161,000 troops in Iraq and commanders plan to take stock of force levels in the summer, after completing a round of withdrawals that should leave around 140,000 U.S. military personnel in the country.

In the wide-ranging speech, Gates also discussed Iran. He said Tehran supports terrorism, "is a destabilizing force throughout the Middle East and Southwest Asia and, in my judgment, is hell-bent on acquiring nuclear weapons."

The Iranian government says its nuclear program is intended only to generate energy while the United States and other Western countries say Tehran wants to be able to build atomic weapons.

A U.S. intelligence assessment in December said Iran halted its nuclear weapons program in 2003.

"Another war in the Middle East is the last thing we need and, in fact, I believe it would be disastrous on a number of levels," Gates said, according to the prepared text of his speech released by the Pentagon.

"But the military option must be kept on the table, given the destabilizing policies of the regime and the risks inherent in a future Iranian threat -- either directly or through proliferation." (Reporting by Andrew Gray; editing by Mohammad Zargham)

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