Al-Qaida's front group in Iraq reaffirmed its support Monday for the terror network's second-in-command, Ayman al-Zawahri, a week after U.S. commandos killed Osama bin Laden.

Al-Qaida has not yet announced a successor and there is some uncertainty about whether al-Zawahri will indeed take over for bin Laden.

In a statement posted on an insurgent website Monday, the Islamic State of Iraq also taunted President Barack Obama as "the rat in the black house" and said he should remain fearful of bin Laden's promise to threaten the United States.

"The martyrdom of the sheik (bin Laden) will increase the determination and steadfastness among his brotherly mujahideen," read the statement. "To our brothers in al-Qaida, first among them sheik Ayman al-Zawahri and his brothers in the leadership of the organization, may God reward you and grant you patience for this loss."

The statement amounts to an official letter of condolence for bin Laden, who was killed in a U.S. strike last week on his compound in northern Pakistan. It was signed by Abu Bakr al-Husseini al-Baghdadi, a pseudonym for the anonymous new leader of al-Qaida's front group in Iraq.

The Egyptian-born al-Zawahri was long bin Laden's deputy, but has clashed in the past with al-Qaida's front group in Iraq. Both bin Laden and al-Zawahri urged followers to focus attacks against U.S. and Israeli interests, and sharply condemned the network's Iraqi wing for targeting Shiite Muslims during the years that Iraq teetered on the brink of civil war.

Al-Qaida is generally comprised of Sunni Muslims.

The new statement did not call for specific attacks on the U.S. but said Obama should be mindful of bin Laden's 19 operatives who attacked the World Trade Center and the Pentagon on Sept. 11, 2001.

"And now the same fear and terror after his death will hurt you as his legacy, and will be a hard burden on you, spoiling your lives and threatening your security and consuming your economy," the statement said. "It is the right of Osama bin Laden's men to fulfill his oath."

Al-Zawahri has been critical of Obama, even releasing a message that referred to Obama as a "house Negro," a slur for a black subservient to whites.

Al-Qaida in Iraq has by and large confined its attacks to its home country. In a second statement Monday, it claimed responsibility for a suicide bomber who last week killed 20 police officers in the Iraqi town of Hillah by ramming the explosives-packed vehicle he was driving into a barricaded security compound.

It was the second major deadly blast in Iraq of the week, and suggests that al-Qaida-linked groups in Iraq remain a threat despite the death of bin Laden, their ideological patron. Violence has dropped nationwide since 2008, but militants are still capable of carrying out bombings and launching attacks against Iraqi and U.S. forces.

Also Monday, a roadside bomb near a police patrol in central Baghdad killed two passers-by. Counterterror experts believe roadside bombs in Iraq generally are a hallmark of al-Qaida and other Sunni insurgents.