Coalition troops, led by the United States Army's V Corps, are now near Karbala, nearly 100km south of Baghdad, British Prime Minister Tony Blair told the House of Commons on Monday.

"They will encounter the Medina Division of the Republican Guard," he said. "This will plainly be a crucial moment."

"The two main bridges over the Euphrates south of Baghdad have been taken intact. This is of critical significance," he added.

The prime minister's statements came as explosions and anti-aircraft fire lit the night skies over Baghdad.

The first of the four waves of bombings destroyed, among other targets, an Iraqi air force building, according to CNN.

Mosul city in the north of the country was also bombed, it said.