We haven't seen this before, to resort to a suicide attack

Pakistani spokesman Rashid Qureshi

The three men are believed to have bought the car used in the attack.

About 300 suspected militants have been detained since the attack, which took place in the port city of Karachi on Wednesday.

Most of those detained belonged to banned groups including Kashmiri separatists and sectarian organisations.

Investigators are examining the possible involvement of Osama Bin Laden's al-Qaeda network.

Extremism

Police said the suicide vehicle, a 1976 Toyota Corolla, was purchased from a dealer a day before the attack.

Most of the victims were sitting on a navy bus waiting to go to work when the car exploded next to them.

Police are focusing on the suicide vehicle

They included a number of French technicians working for the Pakistan navy.

"We are preparing sketches of three suspects, who bought the car used in the attack," an investigator told the Reuters news agency.

Interior Secretary Tasneem Noorani said many of those being interrogated had been detained earlier in the year but were freed after 90 days according to the law.

The bombing - the third attack in less than four months directed against foreigners in Pakistan - has raised tension in the city and caused fear among foreign nationals.

Pakistani President General Musharraf has been a key Western ally in the war against terror and has come down heavily on extremist groups.

"Certainly the extremist elements in Pakistan were unhappy and al-Qaeda is very unhappy that Pakistan has joined very strongly in this war against terrorism," said a spokesman for the military government, Major General Rashid Quereshi.

"We haven't seen this before, to resort to a suicide attack. This is a new act of extremism," he added.

However, Interior Secretray Noorani says it is still difficult to establish if the al-Qaeda network was involved in the bombing.

Climate of fear

On Thursday, French Defence Minister Michele Alliot-Marie visited the scene of the blast, outside the Sheraton Hotel.

France believes al-Qaeda could be involved

Security has been stepped up around Karachi and the city's foreign residents feel especially vulnerable.

"It was a horrific incident," a Western oil executive told Reuters.

"The likelihood of what happened happening again is clearly a big threat."

The Pakistani interior ministry has placed an advertisment in leading newspapers urging the public to report any suspicious foreign nationals.

French Government officials and members of the Pakistani administration say the al-Qaeda network could be involved.

"There is plenty of evidence to believe that some regrouping of Pakistani and Afghan Taleban has taken place in various Pakistani cities under active al-Qaeda control in recent weeks," a Pakistani security official was quoted as saying in The News daily newspaper.

The United States has condemned the suicide attack on French workers in Pakistan as a "heinous attack" on two of America's closest allies in the war on international terrorism.