Anti-terrorism police told a news conference that the four detainees were members of the northern Iraq-based radical Islamist group Ansar al-Islam, which Washington has tied to Osama bin Laden's Al-Qaeda, it said.

On Thursday, police said they had made three arrests, but the anti-terrorism squad confirmed that four people had been detained and said they had links with nine Turks held since early last month in the northwestern city of Bursa. They too had ties to Ansar al-Islam, it said.

The news conference also heard that firearms and several dozen remote-control bomb detonators had been seized in two districts in eastern Istanbul, Anatolia said.

Turkish police in recent weeks 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 last year.

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

WAR.WIRE