They had boarded a plane in Belgrade early on Wednesday morning.

Mr Mrksic faces the more serious charges. He is accused of crimes against humanity for the alleged killing of at least 200 non-Serbs near the Croatian city of Vukovar in 1991.

Mr Martic is accused of ordering the shelling of the Croatian capital Zagreb in 1995, in which at least seven civilians were killed.

"I am not guilty, I am going to fight for the truth about my people... I am going there proudly," Mr Martic told reporters before he boarded the plane to Amsterdam.

"If that is the court of justice, I will be back here."

Breakaway state

In 1991, former police chief Mr Martic led the Krajina Serb republic as it broke away from Croatia.

The fragmented mini-state survived until 1995, when the Croatian army moved in to retake its lost territories.

Mr Martic has been in hiding for the past four years. His lawyer said he had been living in a small village in Serbia.

In the first years of the Croatian Serb rebellion, Mr Martic was close to Slobodan Milosevic, the former Yugoslav president.

The two fell out after Mr Milosevic - the then president of Serbia, the dominant Yugoslav republic - allowed Croatian forces to retake the territory held by the Krajina Serbs.

It is alleged that Mr Martic then ordered the retaliatory missile attack on Zagreb.

The chief war crimes prosecutor at The Hague, Carla Del Ponte, recently said that the indictment against Mr Martic would be expanded to include specific actions by his troops in Croatia.

City besieged

Mr Mrksic commanded a Yugoslav army unit which besieged and shelled the eastern Croatian city of Vukovar in 1992. He later commanded Croatian Serb troops under Mr Martic.

Martic has accused him of betrayal for leaving Croatia along with his troops, and it is believed the two remain enemies.

So far, only six of 23 war crimes suspects have agreed to demands by the Yugoslav Government that they turn themselves in voluntarily.

Two of the most wanted suspects, Bosnian Serb wartime leader Radovan Karadzic and his military chief Ratko Mladic remain at large.