Men assist a bomb blast victim in Baghdad February 3, 2007. A suicide bomber killed 121 people on Saturday in the deadliest single bomb blast in Baghdad since the 2003 war, when he drove a truck packed with one tonne of explosives into a busy market in a mainly Shi'ite area. [Reuters]

BAGHDAD, Iraq - A suicide bomber driving a truck loaded with a ton of explosives hidden beneath cooking oil, canned food and bags of flour obliterated a Baghdad food market, killing at least 121 people in one of the most fearsome attacks in the capital since the US invasion in 2003.

The explosion Saturday was the fifth major bombing in less than a month targeting predominantly Shiite districts in Baghdad and a provincial city to the south. This one leveled about 30 shops and 40 houses, witnesses said.

The Health Ministry said more than 300 people were injured in the thunderous explosion that sent a column of smoke into the sky on the east bank of the Tigris River. The nearby al-Kindi hospital _ quickly overwhelmed _ began turning away the wounded and directing ambulances to hospitals in the Shiite Sadr City neighborhood.

US Ambassador Zalmay Khalilzad said the bombing was "an example of what the forces of evil will do to intimidate the Iraqi people."

The bombing came just days before American and Iraqi forces were expected to start an all-out assault on Sunni and Shiite gunmen and bombers in the capital.

Only a day earlier, 16 American intelligence agencies made public a National Intelligence Estimate that said conditions in Baghdad were perilous.

"Unless efforts to reverse these conditions show measurable progress ... in the coming 12 to 18 months, we assess that the overall security situation will continue to deteriorate," a declassified synopsis of the report declared.

Emergency workers and civilians wheeled scores of bloodied and mangled victims into the hospitals with intravenous drips already in their arms. Doctors and paramedics were in a frantic triage to save the lives of the most seriously wounded.

"We don't allow big trucks in the market, but the driver convinced us that he had food to deliver for a shop. Once he got inside, he detonated the bomb," said Kamil Ibrahim, a 36-year-old vegetable vendor at the entrance to the market district.

Ibrahim _ wounded in his head, chest and abdomen _ said two of his workers, young men 18 and 19 years old, were killed instantly.

The shopkeeper spoke from a bed in al-Kindi Hospital, where he was rushed in a private car after rescuers wheeled him out of the market on a wooden cart.

Suspicion immediately fell on Sunni insurgents _ al-Qaida in Iraq and allied groups in particular. The militant bombers are believed to have stepped up their campaign against Shiites in the final days before the joint US-Iraqi crackdown in Baghdad. Many saw the operation as a last-chance effort to clamp off violence that has turned the capital into a sectarian battleground.

Suspected Sunni attackers have appeared emboldened in recent weeks after radical anti-American Shiite cleric Muqtada al-Sadr, under pressure from fellow Shiites who dominate the government, ordered the thousands of gunmen in his Mahdi Army militia to avoid American attacks in the coming assault.

In the hours after the explosion, Shiite and Sunni mortar teams traded fire across the darkened city. Two people were killed and 20 wounded in one predominantly Sunni district.

The White House called the bombing an atrocity and said, "Free nations of the world must not stand by while terrorists commit mass murder in an attempt to derail democratic progress in Iraq and throughout the greater Middle East."

Violence shattered the northern city of Kirkuk as well. Eight bombs exploded within two hours, the opening blast a suicide car bomber apparently targeting the offices of the Kurdish Democratic Party of Massoud Barzani, leader of Iraq's autonomous Kurdish region.

Two people were killed in that blast and four nearby homes destroyed. There was no claim of responsibility for the series of bombings in the oil-rich city where Arabs, Kurds and Turkomen all claim ascendance.

Sunni insurgents were seen as likely suspects, however, as many of them have fled to the north of the country in a bid to escape the crackdown in the capital.

Further signs the insurgents were migrating north appeared in Mosul, where insurgent forces fought Iraqi police and soldiers. Police said five insurgents were killed. Police spokesman Brig. Abdul Karim al-Jubouri said fighters abandoned their attack when Iraqi security forces moved in backed by US air power.

In the Baghdad blast, Maj. Gen. Jihad al-Jabiri of the Iraqi Interior Ministry said one ton of explosives ripped through the Sadriyah market.

"There are still bodies under the rubble," he said. In an outburst of frustration and anger he called for the government to "deport (non-Iraqi) Arabs immediately."

The general's comments reflected growing displeasure inside the government of Shiite Prime Minister Nouri al-Maliki with neighboring Syria, which Baghdad charges has done too little to close its border to Sunni militants.

In his second heated verbal attack on Damascus in two days, government spokesman Ali al-Dabbagh said "50 percent of terrorism enters Iraq from Syria, and we have evidence" to prove that.

"The Interior Ministry and the Ministry of State for National Security gave them (the Syrians) evidence about those who are conspiring and are sending car bombs. We gave them the numbers of their apartments and the buildings where they live," al-Dabbagh said on Al-Arabiya satellite television.

The Sadriyah market sits on a side street lined with shops and vendors selling produce, meat and other staples. The market is about 500 meters from a Sunni shrine.

The blast was the deadliest attack in the capital since November 23, when suspected al-Qaida in Iraq fighters hit Sadr City with a series of car bombs and mortars that killed at least 215 people.

Not far from the Sadriyah marketplace, a suicide bomber crashed his car into the Bab al-Sharqi market 12 days ago and killed 88 people.

South of Baghdad, a pair of suicide bombers detonated explosives Thursday among shoppers in a crowded outdoor market in the Shiite city of Hillah, killing at least 73 people and wounding 163.

An Iraqi militant group tied to al-Qaida in Iraq announced, meanwhile, it had launched its own new strategy to counter the coming US-Iraqi crackdown.

In an audiotape posted on a Web site commonly used by the insurgents, a voice purported to be that of Abu Abdullah Rashid al-Baghdadi, also known as Abu Omar al-Baghdadi, the head of The Mujahedeen Shura Council, said the group would "widen the circle of battles" beyond Baghdad.

The US military reported the deaths of five more soldiers _ four in fighting and one of an apparent heart attack. All died Friday.

Iraqi authorities said that 145 people were killed or were found dead Saturday, including those killed in the market bombing. Of the total, 19 were found dumped in the capital, most of the bodies showing signs of torture.