Attacks in Baghdad and a city in northern Iraq killed at least 41 people and wounded dozens more on Thursday, the worst violence since Iraq celebrated the withdrawal of American troops from cities and towns last month.

In the deadliest attack, two suicide bombers detonated explosives in Tal Afar, 40 miles from Mosul, the capital of Nineveh Province.

The first bomber, wearing a vest of explosives, targeted two security officials outside the court that handles terrorism cases. The explosion occurred on Thursday morning in the city’s centre, and as crowds gathered afterward, the second bomber struck. At least 34 were killed in those two blasts and 64 wounded, according to preliminary reports from security officials in the region.

In Baghdad, two separate improvised bombs exploded near a market in Sadr City. Those bombings killed at least seven and wounded 20 others, security officials reported.

The twin suicide bombings in Tal Afar bore the signature the Islamic State of Iraq, the umbrella organisation of groups affiliated with al-Qaeda in Mesopotamia.

The attacks came a day after a reputed leader of the Islamic State of Iraq, whom the Iraqi Government claimed to have captured this year, issued a taped statement calling on Iraq’s Sunnis to join the fight against Shiites and American troops.

The statement, which was released on the Internet and attributed to Abu Omar al-Baghdadi, said attacks on American forces should continue despite the withdrawal from Iraqi cities by US combat troops.

“Even if they are in one spot in the Iraqi desert... every Muslim must fight them until they are kicked out of that spot,” the statement said. It was impossible to verify the authenticity of the statement, but it was posted on a website popular with jihadists. If authentic, it would be the first response from the Islamic State of Iraq since United States combat forces completed the urban withdrawal on June 30.