A German judge on Saturday allowed the continued detention of three men suspected of planning an attack on Iraqi Prime Minister Ayad Allawi (search) during his visit to Berlin this week, while a fourth suspect was arrested in the capital, the prosecutors' spokesman said.

The judge in Karlsruhe issued a formal arrest warrant based on suspicion of belonging to an international terrorist group, allowing authorities to continue holding the men, who were picked up on Friday — hours before Iraqi leader Allawi met German Chancellor Gerhard Schroeder (search).

The men are believed to belong to Ansar-al-Islam (search), which has mounted attacks on U.S. and allied forces in Iraq since last year's U.S.-led invasion.

A fourth suspect, a man with Lebanese citizenship, was arrested in Berlin on suspicion of supporting a foreign terrorist organization and was being questioned by police, prosecutors' spokesman Hartmut Schneider said. Prosecutors did not release the suspects' names.

One of them, the suspected head of an Ansar-al-Islam cell in the southern German state of Baden-Wuerttemberg, has been under investigation by Stuttgart authorities since October 2003, Der Spiegel newsweekly reported.

He was brought to the attention of German authorities by prosecutors in Milan, Italy, after a telephone call with a key terror suspect in Italy, the magazine reported.

Before Allawi's visit, investigators who had the three suspects under surveillance noticed an increase in activity, phone calls and suspicious movements by one suspect that amounted to evidence of plans to attack, prosecutors say.

The suspects' phone calls grew more hectic after initial intelligence led officials to cancel a Thursday night meeting between Allawi and Iraqi exiles in Berlin, prosecutors say.

Ansar al-Islam, which was formed in the Kurdish parts of Iraq, is believed to include Arab Al Qaeda members who fled the U.S.-led invasion of Afghanistan in late 2002.

Jordanian terrorist Abu Musab al-Zarqawi (search), whose followers in Iraq have claimed responsibility for numerous car bombings and beheadings of foreign hostages, including three Americans, is believed to have played a key role in the group after fleeing Afghanistan.

The United States has offered a $25 million reward for al-Zarqawi's capture — the same amount as for Osama bin Laden.

Allawi said the threat was part of his everyday life, the Munich-based Sueddeutsche Zeitung daily reported Saturday.

"But, we will not give up, even when a few terrorists want to force us to," the newspaper quoted him as saying at a Friday evening event.

Allawi left Germany later Friday for Russia, wrapping up a three-nation trip.