At least 80 people were killed on Monday in the northern Iraqi city of Kirkuk in a coordinated attack by a suicide truck bomber in a crowded market and a separate car bomb parked on a busy street, police said.

South of Baghdad, thousands of U.S. troops swooped on a suspected al Qaeda in Iraq safe haven used to reinforce militants fighting in the capital, the military said.

Iraqi police said 136 people were wounded in the Kirkuk blasts and warned that the death toll could rise further.

A Reuters cameraman on the scene described carnage after the truck bomb in the market, near an office of the Patriotic Union of Kurdistan, the party of Iraqi President Jalal Talabani.

The explosion scattered bodies across the market, set dozens of cars on fire and trapped passengers on a bus where they burned to death, the cameraman said.

The car bomb exploded in a commercial area called Iskan, near shops and a bus garage, police said. The two blasts came within minutes of each other, police said.

U.S. and Iraqi forces have launched a series of big security clampdowns since the last of 28,000 extra U.S. troops ordered to the country by U.S. President George W. Bush arrived a month ago.

They aim to thwart violence between majority Shi'ites and minority Sunni Arabs which has pushed the country towards full-scale civil war, while winning time for Shi'ite Prime Minister Nuri al-Maliki to deliver key power-sharing laws.

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.

But Bush says he will not alter course before a September review from General David Petraeus and Ambassador Ryan Crocker, his top two personnel in Iraq.

The operation south of the capital, called Marne Avalanche, aims to stem the flow of weapons and militant fighters into the southern part of Baghdad, where U.S. and Iraqi forces are already fighting hard to clear them out.

In pre-dawn raids, helicopter-borne troops swept into an area the U.S. 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 U.S. 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.

PHOTO CAPTION

U.S. soldiers fire with their combat rifles during an exercise inside Camp Victory, near Baghdad July 14, 2007. (Reuters)

Reuters