The deadly bombings came hours after US aircraft and artillery blasted suspected guerrilla hideouts in Baghdad in an attempt to flush guerrillas from the city of five million.

A roadside bomb ripped through a US military convoy driving on a highway near the town of Samarra, 100 km (60 miles) north of Baghdad, killing three soldiers and destroying their vehicle, the military said.

The deaths brought to 205 the number of US soldiers killed since Washington declared an end to major combat in Iraq on May 1.

Civilian and military officials of the US-led occupation forces have warned that diehard insurgents would launch spectacular attacks during the holiday season.

The US meanwhile issued a new alert call after receiving was a "specific" threat of an attack in the Gulf state of Bahrain. Security was stepped up across the US for Christmas and the New Year.

In the northern Kurdish city of Arbil, a suicide car bomb blew up at the entrance of the interior ministry headquarters, killing the bomber, two policemen guarding the facility, a passerby and a 13-year-old girl. A Kurdish official said more than 100 people were wounded.

The official blamed "extremists who came from outside Iraq", without elaborating.

The US-backed government's building was damaged in the blast and a number of cars were destroyed. The force of the explosion left a huge crater on the road.

Dozens of explosions and heavy machinegun fire had echoed overnight across Baghdad as aircraft buzzed overhead in one of the capital's heaviest bombardments since the end of the war.

Captain Jason Beck, spokesman for the First Armoured Division that controls Baghdad, said Operation Iron Grip was aimed at guerrillas who might use the holiday season to step up attacks on coalition forces.

"The purpose of the operation is to attack, kill or capture individuals (launching attacks) against coalition activities," Beck told Reuters. "Enemies will use the holidays to attack for psychological reasons.

"This is obviously a special time of the year for all soldiers, so we must remain vigilant."

US-led occupation forces in Iraq have tightened security in Baghdad and other hotspots.

The mainly Kurdish north has seen less violence than the rest of Iraq since US-led forces deposed Saddam Hussein in April, but several car bombs and assassinations have targeted US troops and Iraqis working with them in the region.

The US government on Sunday raised its nationwide security level from yellow, denoting an "elevated" risk of attack, to orange, or "high" risk, amid concerns of an impending attack on American territory.

The decision was triggered by fresh intelligence indicating Osama bin Laden and his top deputy personally approved a plan for a new terrorist attack, NBC News reported.

Citing unnamed US officials, the television network said US intelligence agencies had gathered detailed information about the plan, according to which al-Qaeda operatives will seek to hijack foreign airliners and fly them into targets in the United States.

As a result several US airports, particularly Newark International Airport in New Jersey, have found themselves under intense scrutiny, NBC News said.

US officials and terrorism experts said al-Qaeda seems particularly interested in Tappahannock, Virginia, a tiny town of 2,016 people with no military base or major infrastructure, the report said.

Such an attack would be intended to generate widespread fear that no one, even residents of small towns, was safe in the United States, NBC News quoted US officials as saying.

White House spokesman Scott McClellan said new intelligence points to the possibility of a spectacular attack that could "rival or exceed the scope and impact of those we experienced on September 11."

President George W. Bush has urged Americans to go ahead with their shopping and other Christmas activities. But security has been toughened at US borders, airports, shopping centers and nuclear facilities to confront the increased risk.