BAGHDAD – A bomb hidden inside a toilet struck a Sunni mosque south of Baghdad on Saturday, injuring two people, according to Iraqi police, the latest in a wave of attacks against Islamic sites of worship.

A series of bombings targeted Shiite mosques a day earlier during Friday prayers in Baghdad, killing at least 29 people.

U.S. officials have warned they expect insurgents to try to re-ignite retaliatory sectarian violence, which nearly pushed the country to the brink of civil two years ago. Violence has dropped sharply over the past two years following a U.S. troop surge in 2007, a Sunni revolt against al-Qaida in Iraq and a Shiite militia cease-fire.

The continuing incidents, however, have raised concerns about the capabilities of Iraqi forces, which assumed responsibility for security the cities a month ago as American troops pulled back to bases outside urban areas.

The Sunni Iraqi Islamic Party denounced the "blind wave of violence" and blamed Iraqi security forces for failing to protect the people.

According to police, no one has been arrested or charged in any of the explosions.

U.S. officials have said they were optimistic that Shiite groups would exercise restraint in the face of the attacks against their mosques. In the past bombings blamed on Sunni insurgents were followed by mortar attacks and execution-style killings against Sunnis by Shiite gunmen.

Sunnis also have been targeted by their co-religionists, such as al-Qaida in Iraq and other Sunni insurgents, who seek to undermine their cooperation with the Americans and the Shiite-led government.

The al-Sahaba mosque was bombed in the pre-dawn hours in the town of Jurf al-Sakr, 40 miles (65 kilometers) south of Baghdad, said a police official, who spoke on condition of anonymity because he wasn't authorized to speak to the media.

Also Saturday, Al-Jazeera television broadcast an audio clip purportedly from a former top deputy of Saddam Hussein calling on Sunni insurgents to unite under one political umbrella.

Izzat Ibrahim al-Douri, whose whereabouts are unknown, is believed to play an important role in financing Sunni insurgents and has a $10 million bounty on his head.

In the clip, al-Douri said the united groups should only negotiate with the United States after it pulls out from Iraq and recognizes the insurgents as the legitimate representative of Iraqis.

It was not clear if al-Douri's appeal was a reaction to recent reports that indicate Washington and Turkey have held reconciliation talks with Sunni insurgents.

Separately, a senior Iraqi official said three members of Iraq's presidential guard have been arrested for allegedly carrying out a daring, pre-dawn bank robbery in Baghdad that left eight security guards dead.

Iraqi Interior Ministry spokesman Maj. Gen. Abdul-Karim Khalaf said two officers and a soldier in the presidential brigade were arrested. He said the officers were "corrupt" and might have had a source inside the bank. No doors or windows were broken in the robbery and the bank was robbed just after it had received a large transfer of cash.

Khalaf said it was a rogue act, and denied speculation it was an attempt to raise funds for an insurgent group. He noted it was also a highly professional, cold-blooded heist. After tying up and executing the guards, the robbers used blowtorches to get inside the bank's safe.

The robbery netted around $7 million — all of which has been recovered, he said.