BAGHDAD, Iraq - A suicide bomber attacked Iraqis applying for jobs as policemen today, killing nine and wounding 21, and U.S. officials said a top deputy of the leader of al-Qaida in Iraq was killed over the weekend.

NATO Secretary-General Jaap de Hoop Scheffer visited Baghdad to review the alliance's training mission for the Iraqi military. The unannounced visit was de Hoop Scheffer's second trip to Iraq. He was accompanied by the alliance's supreme commander for operations, U.S. Gen. James L. Jones.

NATO has been training a small group of senior Iraqi military officers and is planning to expand that mission to include higher ranks of Iraq's armed forces.

The blast occurred in Baqouba, 30 miles north of Baghdad, said the officer, who spoke on condition of anonymity because of concerns about his security. A man wearing explosives under his clothes blew himself up in a building where Iraqis were applying to join the country's Quick Reaction Police Force.

The attack raised to at least 61 the number of people killed in the past three days in Iraq, less than a month before a national referendum on Iraq's draft constitution.

Politicians and insurgents in Iraq's Arab Sunni minority have urged Iraqis to boycott the referendum or vote "no." They believe a charter will fracture the country and seal the domination of the Shiite majority.

On Monday, American and Iraqi officials tried to rally Sunni support for the referendum by releasing 500 detainees from the notorious Abu Ghraib prison outside Baghdad to mark the coming Islamic holy month of Ramadan.

But if two-thirds of voters in any three of Iraq's 18 provinces reject the charter, a new government must be formed and the process of writing a constitution starts over.

On Monday, insurgents dragged five Shiite Muslim schoolteachers and their driver into a classroom, lined them against a wall and gunned them down.

Also Monday, a suicide attack and roadside bombings killed 10 Iraqis and three American soldiers.

In Washington, U.S. defense officials said that Abdullah Abu Azzam, a leading deputy to Abu Musab al-Zarqawi, al-Qaida's leader in Iraq, was killed this weekend. The officials spoke on condition of anonymity, citing the sensitivity of the information.