WASHINGTON, June 5 (Reuter) - The United States said on Monday it will move 3,500 troops and about 100 helicopters to Italy to support any U.N. retreat from Bosnia and officials raised hopes a U.S. pilot shot down in Bosnia might be alive.

Defence officials said attack and transport helicopters and troops would be moved from Germany to Italy beginning this week under President Bill Clinton's promise to support a massive NATO plan to aid any future U.N. withdrawal from Bosnia.

But one of the officials, who asked not to be identifed, refused to rule out use of the helicopters and troops if an emergency call came from harried U.N. troops to be moved from one position to another within Bosnia.

Clinton jumped to the defence of U.S. policy in Bosnia on Monday night against a chorusof criticism from both Republicans and Democrats.

"I'm not happy with everything that's happened in Bosnia. I wish there were some clear-cut answers, Clinton said in an interview with Cable News Network.

But he said by sheer numbers of casualties alone, American policy has been successful, comparing 130,000 deaths in Bosnia in 1992 to last year's less than 3,000.

The president said American troops should not and would not be involved on the ground in Bosnia but if U.N. peacekeepers "get stranded and they are in desperate conditions I'd be inclined to help them."

General Ronald Fogelman, the air force chief of staff, meanwhile, said on Monday faint electronic beeper signals, possibly from a pilot's emergency beacon, had been received by searchers over northern Bosnia.

A U.S. F-16 was shot down by an anti-aircraft missile over Bosnian Serb-controlled territory Friday and, until Monday, the U.S. Defence Department had said there was no evidence the pilot survived the crash.

Fogelman and another senior defence official, who asked not to be identified, told reporters the signals had given hope to U.S. rescue teams.

"For the last 20 to 24 hours we have been receiving beeper signals which is encouraging," the official told reporters. "We are doing our utmost to locate these signals. The signals alone are not enough to indicate the pilot is alive but it is encouraging."

The official also said there had been reports from Serbia that indicated a parachute had been found on the ground but they had not been confirmed.

U.S. officials, meanwhile, remained on the defensive about a series of changes to Clinton's Bosnia policy but said they were not prepared to make another change and lift an arms embargo against the Bosnian government.

Both Democratic and Republican members of Congress have expressed concern the United States will become bogged down in the Bosnia civil war and the administration has changed positions several times on exactly how U.S. troops would be used to aid troubled U.N. forces there.

Under a NATO contingency plan, as many as 60,000 allied troops -- about 25,000 of them Americans -- could be used to withdraw the U.N. force if the United Nations decides on a full-scale removal of its 23,000 peacekeepers from Bosnia.

Officials said the initial movement of Apache attack helicopters, transport helicopters and 3,500 troops could be followed soon by more movements of U.S. troops near Bosnia.

White House spokesman Mike McCurry told a news briefing before Pentagon officials spoke on Monday the United States was doing "everything conceivable to secure credible information about the U.S. pilot's whereabouts..."

He said Ratko Mladic, the head of Bosnian Serb forces, had made "conflicting statements" to U.N. authorities about the pilot, whose plane was shot down as it patrolled over Bosnia to enforce a U.N. "no-fly" zone.

"I'm just not going to speculate on information concerning the fate of the pilot until we have concrete information that we believe is clear that we can pass on," he said.

Bosnian Serb officials said they had captured the American but have not produced any evidence and have not clearly indicated if he is alive.