At least 85 people were killed on Monday by a suicide truck bomb in the volatile Iraqi city of Kirkuk, some of them trapped on a bus where they burned to death, according to a witness.
Police also said 180 people were wounded and they warned the death toll could rise from the blast that heightened tension in the northern city, shared by Kurds, Turkmen, Shi'ite and Sunni Arabs, that plans to hold a crucial referendum on its future.
The blast, near an office of the Patriotic Union of Kurdistan of Iraqi President Jalal Talaban, was one of several attacks in Kirkuk that is supposed to vote this year on whether to join semi-autonomous Kurdistan.
"Tens of houses and shops were totally destroyed by the power of the explosion," said Police General Torhan Abdul Rahman, the city's deputy chief of police.
Dozens of cars were set on fire and passengers were trapped on a bus where they burned to death, said a Reuters cameraman at the scene.
Police said 25 of the wounded were in a critical condition and many bodies might still be buried in the rubble.
The truck detonated minutes apart from a car bomb in a busy Kirkuk shopping area that wounded two people, police said.
A police officer was killed and four officers were wounded soon after, when a parked car bomb exploded in southern Kirkuk, police said. A fourth car bomb was discovered and made safe.
South of Baghdad, thousands of US troops swooped on a suspected Al-Qaeda in Iraq safe haven used to reinforce militants fighting in the capital, the military said.
US and Iraqi forces have launched a series of big security clampdowns since the last of 28,000 extra US troops ordered to the country by US President George W Bush arrived last month.
They aim to thwart violence between majority Shi'ites and minority Sunni Arabs which has pushed Iraq towards civil war, while winning time for Shi'ite Prime Minister Nuri al-Maliki to deliver key power-sharing laws.
But the devastation in Kirkuk, 70 km (50 miles) north of Tuz Khurmato where a massive truck bomb killed at least 130 on July 7, shows the scale of the challenge. Time is pressing.
Many Americans want their soldiers to come home soon and senior members of Bush's own Republican Party have broken ranks to call for a change of war strategy.
Bush says he will not alter course before a September review to US lawmakers from General David Petraeus and Ambassador Ryan Crocker, his top two personnel in Iraq.
The operation, called Marne Avalanche, aims to stem the flow of weapons and militant fighters into the capital, where US and Iraqi forces are already fighting hard to clear them out.
In pre-dawn raids, helicopter-borne troops swept into an area the US military said was an Al-Qaeda safe haven around the Euphrates river valley, 35 km (22 miles) south of Baghdad.
The terrain, criss-crossed with an extensive canal system, has been the location of fierce fighting between US forces and militants in the past and at least one air strike was called in during the early hours of the operation, a spokeswoman said.
US commanders says Iraqi security forces are a long way from being able to keep the peace without US help and a senior officer told the New York Times that success would not be in sight before spring next year.
"It is going to take us through the summer and fall to deny the enemy his sanctuaries ... and then it is going to take us through the first of the year and into the spring" to secure these gains, it quoted Major General Rick Lynch as saying.