US-led coalition forces bombers started bombardment in and around Baghdad at 7:30 a.m. local time (0230 GMT) on Tuesday, eyewitnesses told Xinhua.
The air raids continued for about one hour and were most probably conducted by B-52 bombers which were seen flying over the Iraqi capital.
No details about the damage and casualties were immediately available.
More than three hours later, new heavy bombardment shook Baghdad again at 10:15 a.m. local time (0715 GMT) as loud explosions were heard from different parts of Baghdad, particularly from the northern part of the city.
Clouds of smoke from burning fuel trenches blackened the sky.
US or British warplanes could be heard roaring at high altitude over the capital, but could not be seen amid the dark clouds.
The morning bombardment came after overnight airstrikes poundedthe Iraqi capital.
A Drone unmanned plane of the US coalition forces was shot down in Baghdad on Tuesday morning, the Iraqi television reported.
It was the third Drone plane shot down by Iraqi anti-aircraft artillery since the war began on Thursday.
Meanwhile, a column of about 4,000 US marines had reportedly crossed the Euphrates River in the city of Nasiriyah on Tuesday.