Reuters reporter Joe Logan in Kurdish-held northern Iraq saw the blasts amid anti-aircraft fire over the city on Friday night.

He was on high ground with a view of Kirkuk, some 20 miles away in government held territory.

The huge explosions and anti-aircraft fire were the second exchange following big bursts of fire earlier. Lights from the city were no longer visible and it was unclear whether that was due to the smoke or damage to electricity supplies.

The northern Iraqi city of Mosul was also hit by air strikes, al-Jazeera television said in a live report from the city.

A Reuters correspondent nearby also reported anti-aircraft fire over the city.

The attack was reported at almost the same time as fresh explosions were heard near Baghdad, and as a U.S. official said a major air war against Iraq had been launched.