Twin Explosions here in Tal Afar! Please Pray For these families.

(CNN) At least 35 (Now estimated at 40 and growing) people were killed and 65 wounded in a double suicide bombing in northern Iraq on Thursday morning, according to the Nineveh Military Operations Command.

The bombers struck the city of Tal Afar in Nineveh province, where political tensions have recently increased between Arabs and Kurds. Violence erupted in Nineveh on Wednesday as well.

The first bomb struck the house of a security official working in counterterrorism operations, the official said. The second bomb detonated just minutes later, after people had gathered at the scene in a predominantly Shiite Turkmen area of the city.

The Tal Afar bombings occurred a day after attacks on mainly Shiite targets in Nineveh province -- including a car bomb in a Turkmen area of the provincial capital, Mosul, -- left at least 19 people dead and dozens wounded.

Despite a drop in violence in Nineveh's provincial capital, Mosul, the city remains a challenge. There are daily reports of attacks.

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*The 130,000 U.S. troops who remain are now tasked with supporting Iraqi troops and police, and will require Iraqi permission to launch operations in the cities. Most are American Police Transition Teams that work in the cities with the Iraqi Police.

(More from the news reel) The bombers in Tal Afar targeted two brothers working for Iraqi security forces in Tal Afar in an attack against a building used as a court annexe to interrogate suspects in "terrorist attacks," police colonel Khaled Omar told AFP.

They blew themselves up minutes apart around 7.30am (0430 GMT), with the second explosion engulfing civilians who had gathered to help victims of the first blast, a security official said.

The violence was the worst to strike the conflict-hit country since US forces pulled out of towns and cities on June 30 under a landmark accord between Baghdad and Washington.

Dr Fathi Yassin at Tal Afar hospital said 34 people were killed and 61 wounded in the attack, which came a day after two car bombs exploded in the main northern city of Mosul, killing 12 people and wounding dozens more.

Neither of the two brothers targeted by the Tal Afar bombers were killed, the security official said, but police and hospital sources said the casualties included women and children.

Another six people were killed and 24 hurt when two bombs went off in a market in the mainly Shiite district of Sadr City in Baghdad, police and hospital sources said.

A roadside bomb targeting the convoy of the Iraqi Central Bank's governor, meanwhile, resulted in a civilian being killed and five people, including two members of the security forces, wounded in the capital's Karrada business district.

Tal Afar is a mostly Turkmen town between Mosul and the Syrian border and has often been the target of violence.

In March 2007, it was hit by one of the deadliest single attacks in Iraq since the US-led invasion of 2003 when a suicide truck bomb killed more than 150 people.

The spate of bombings comes just over a week after US troops pulled back from urban centres under an agreement that paves the way for a complete American military withdrawal by the end of 2011.

The four weeks leading up to the US withdrawal witnessed the highest death toll in the country in 11 months, according to official figures.

A total of 437 people, including 372 civilians, were killed in June, according to figures compiled by government ministries -- the highest toll since since July 2008.

Prime Minister Nuri al-Maliki had warned last month that insurgents and militias were likely to step up attacks in a bid to undermine confidence in Iraqi security forces.

US Vice President Joe Biden, who has repeatedly voiced concern about lingering feuds between Iraq's various sects being a roadblock to political progress, was last week rebuffed on the issue when he visited Iraq.

"We don't want other parties to interfere in this matter because it will cause complications," government spokesman Ali al-Dabbagh said on state television, referring to the country's national reconciliation process.

"(Joe Biden) has to convey to President (Barack) Obama the common desire of Iraqis to solve their problems together," he said.

Baghdad's comments came a day after Biden warned of a "hard road ahead if Iraq is going to find lasting peace and stability," alluding to the need to bolster trust between different ethnic and religious groups.

Maliki, who will visit Washington on July 21, said that the US troop pullback signalled that the two countries had "entered a new phase."