Sri Lanka imposed a state of emergency Saturday, following the killing of the country's foreign minister, a close advisor to the president.

Troops set up roadblocks in Colombo and began an intensive search for the assassins. At least two people have been detained, but no further details about the suspects are available.

Military officials in Colombo say Tamil Tiger rebels are prime suspects in the sniper killing of Foreign Minister Lakshman Kadirgamar. President Chandrika Kumaratunga, who is appealing for calm and restraint, has not specifically blamed the rebels for the attack.

The rebels deny they had any part in the killing. They contend the shooting was linked to a military faction in Sri Lanka with a "hidden agenda" aimed at sabotaging a cease-fire that has been in effect for more than three years.

Foreign Minister Kadirgamar, a Tamil who had been the target of many previous death threats, was shot in the head and chest late Friday in the garden outside his heavily guarded home in the capital. He died early Saturday at a hospital.

U.S. Secretary of State Condoleezza Rice condemned the assassination as a "vicious act of violence," and she urged the people of Sri Lanka not to let the 73-year-old foreign minister's "senseless murder" trigger further violence, or destroy the 2002 cease-fire.

U.N. Secretary-General Kofi Annan also expressed his shock and sadness. Norwegian diplomats who helped negotiate an end to Sri Lanka's long civil war say the foreign minister's death is a major setback for the island nation's fragile peace process.

Some information for this report provided by AP, AFP and Reuters.