Anti-terrorism police told a news conference the four, all Turks, were thought to be members of the northern Iraq-based radical Islamist group Ansar al-Islam, which Washington has accused of having ties with Osama bin Laden's Al-Qaeda network, it said. They did not say when they had been arrested.

Two of the four were remanded in custody by a court in Istanbul on Friday on suspicion of "belonging to and supporting an illegal organisation", Anatolia said.

The anti-terrorism squad said the four suspects had been arrested during raids in two districts in eastern Istanbul during which firearms and several dozen remote-control bomb detonators had been seized, Anatolia said.

The police said the four had links with nine other Turks who have been in detention since early last month in the northwestern city of Bursa and who are also believed to have ties with Ansar al-Islam.

In recent weeks the Turkish police have cracked down on both left-wing and Islamist underground groups in preparation for the NATO summit on June 28-29, which will be attended by US President George W. Bush and 45 other heads of state or government.

Security fears have been running high in Istanbul, a sprawling metropolis of more than 10 million people, since suicide bombings against two synagogues, the British consulate and a British bank last November claimed 63 lives and wounded hundreds more.

Authorities blamed the carnage on local Islamist militants tied to Al-Qaeda.

Founded in December 2001, Ansar al-Islam used to control a small enclave in northeastern Iraq before it was crushed by US forces in late March 2003.

On March 22, the United States added Ansar al-Islam to its official list of "terrorist organisations", saying it was linked to Al-Qaeda and has mounted attacks against US-led forces in Iraq.

WAR.WIRE