BAGHDAD — Iraq's prime minister appealed for national unity on Saturday and the country's vice president said he was worried about deteriorating security after more than 250 people were killed in the week before a U.S. withdrawal from cities.

The Shiite prime minister, Nouri al-Maliki, blamed a series of bombings on the remnants of al-Qaida in Iraq and said they were aimed at triggering violence between Shiites and Sunnis.

"Today we are in need of unity, as they have shown their teeth against us," al-Maliki said of the extremists responsible for the attacks. "Our system falls when we return to sectarianism."

Nearly all the bombings and deaths in the past week have targeted Shiite areas, including the two deadliest attacks — a June 20 bombing that killed 82 people outside a mosque in northern Iraq and another in the Shiite enclave of Sadr City that killed 78.

Iraq nearly slipped into civil war two years ago and tens of thousands of people died in attacks between Sunni extremists such as al-Qaida and Shiite militias and death squads. It was brought back from the brink by a huge inflow of U.S. troops in 2007 in what became known as the "surge."

As part of an apparent effort to deflate sectarian tensions, the United States late Friday released its most important Shiite prisoner, a key aide to anti-American cleric Muqtada al-Sadr. Shiites have been complaining that hundreds of prisoners, many of them militiamen and followers of al-Sadr, remain behind bars.

Abdul-Hadi al-Daraji, one of al-Sadr's closest political advisers, was arrested in January 2007 at a mosque in Baghdad's eastern Shiite district of Baladiyat. He was handed over by U.S. forces to Sami al-Askari, a senior aide to al-Maliki, inside Baghdad's Green Zone and immediately released.

"The only thing I can say is that I'm released," al-Daraji told The Associated Press by phone. "I can't say more for the time being."