By Andrew Kelly

THE HAGUE, April 24 (Reuter) - The U.N. tribunal for former Yugoslavia, sending out its toughest signal yet that it means business, named Bosnian Serb leader Radovan Karadzic and two other senior Bosnian Serbs as war crimes suspects on Monday.

Bosnian Serb military commander Ratko Mladic and the former head of secret police, Mico Stanisic, were also named by the tribunal as suspected war criminals.

"My office is currently investigating the question of responsibility of these prominent individuals for genocide, murder, rape, torture and the forced removal of many thousands of civilians from large parts of Bosnia and Herzegovina..." chief prosecutor Richard Goldstone told a news conference.

The Bosnian Serb leadership, based in Pale, was also being investigated in connection with the siege of Sarajevo and armed attacks on aid convoys and U.N. peacekeepers, he said.

In Bosnia itself, the U.N. expressed fears the announcements might jeopardise attempts on the ground to extend a shaky truce due to expire on May 1.

Goldstone said the investigations were at an advanced stage and could lead to formal charges by the end of this year.

If indictments are issued, Bosnian Serb leaders would face arrest if they travelled abroad, as Karadzic has done several times for peace talks in Geneva.

Goldstone wants the tribunal to ask Bosnia-Herzegovina to suspend its own legal proceedings against Karadzic and hand the case over to the tribunal, a procedure known as deferral.

"The deferral applications reflect my strategy, which is to indict those in leadership positions, both civilian and military, who are responsible for serious violations of international law," Goldstone said.

The Bosnian government has already indicated that it will agree to the tribunal taking over the investigation and prosecution of the Bosnian Serb leaders.

Three of the tribunal's 11 judges will convene on May 9 to consider Goldstone's request.

The tribunal, set up by the U.N. Security Council in May 1993, is preparing itself for the first international war crimes trials since those held in the aftermath of World War Two.

It was expected to take custody later on Monday of its first suspect, Bosnian Serb Dusan Tadic, who is charged with murdering, raping, beating and torturing Moslems and Croats during an ethnic cleansing campaign in northwest Bosnia in 1992.

Tadic, arrested in Germany early last year, will make a first appearance before the tribunal in the next few days. His trial is expected to begin by the summer.

Separately the tribunal said on Monday it was investigating the mass murder of Moslems by Bosnian Croats inthe Lasva river valley region of central Bosnia - the first time it has published details of alleged atrocities by non-Serbs.

Goldstone has said his investigations have revealed a systematic policy of ethnic cleansing and he has pledged to act against leaders who knew of atrocities and who failed to prevent them or to punish those who committed them.

So far the tribunal has indicted 22 people, all Serbs, of whom the most senior is Zeljko Meakic, the commander of the notorious Omarska detention camp, who is charged with genocide.

But it has managed to gain custody of only one suspect, Tadic. The rest are believed to be at large in Serb-controlled areas of Bosnia or in Serbia itself.

In Bosnia on Monday, U.N. officials were privately describing the timing of The Hague announcement as unfortunate, coming at a time when they have only the slimmest of chances of persuading the Serbs and the Bosnian government to prolong their four-month truce.

"We hope that decisions made in The Hague will not affect the peace process..," said U.N. spokesman Alexander Ivanko.