A series of bombs that appeared to target two churches and a high-traffic neighbourhood in the northern Iraqi city of Mosul detonated Tuesday, killing four people and wounding as many as 40.

According to reports, the first bomb went off on Tuesday morning outside a church in western Mosul and the second explosion came less than 10 minutes later as people gathered at the site of the initial blast.

Across the city, in northern Mosul, a bomb exploded near another church, breaking windows and damaging the church's gates. No injuries were reported in the third blast.

On Monday, Iraqi Defense Minister Abdul-Qader al-Obeidi acknowledged shortcomings in the security forces but said insurgents had changed their tactics amid the US troop withdrawal.

"The nature of terrorism has changed, and terrorists are conducting attacks that aim to inflict the largest casualties," he said, according to The Associated Press.

Although al-Obeidi did not indicate what he meant by changing tactics, AP noted that much of the recent violence has targeted government institutions, as opposed to violence that appeared designed to spark tensions between Shia Muslims and Sunni Muslims.

While the bombings raises tough questions for Iraqi Prime Minister Nouri al-Maliki about the ability of country's security forces ahead of next year's planned withdrawal of US combat troops, senior US military officials have defended the Iraqi forces' efforts.

"It would be tough for any country, any government to prevent these kinds of attacks," Lt Gen Charles Jacoby, the number two US commander in Iraq, said last week.

The US says it plans to keep the bulk of its 120,000 forces in Iraq during the country's March 7 elections to counter violence, but plans to withdraw all but 50,000 troops from Iraq by the end of next August.

The US military plans to leave the country entirely by December 2011.