The al-Qaida organisation in Iraq said in an internet statement it carried out the bomb attack.
"A lion in a martyrdom brigade of al-Qaida in Iraq carried out a heroic operation in Rabia with an explosive belt. He entered a centre of National Guard volunteers, shouted: 'There is no god but God', then exploded," said the statement posted on a site often used by armed groups in Iraq.
It was not possible to authenticate the claim.
Police said the attack occurred on Friday outside a municipal building in Rabia, a town 80km northwest of Mosul, close to the Syrian border.
Officials said the attack occurred in the midst of recruits who were undergoing training in a security-controlled area and that some of the guards might have knowingly allowed the attacker to enter.
"An initial toll put the number of those killed at 25 and the number of those wounded at 35," said police General Said al-Juburi, speaking from Mosul.
The victims were young men waiting to sign up to join the army.
The attack occurred as the United States has placed new urgency on the training of Iraqi soldiers and police in the hope that they can assume greater security responsibility so US and other foreign troops can begin going home next year.
Iraq's army and police recruits
are targets for constant attacks
Police and army recruits are frequently targeted by armed groups.
Focus on Mosul
In recent months, attacks by bombers strapped with explosives, as opposed to driving explosives-laden cars, have become more common.
Mosul, Iraq's third largest city, and areas to its west have been a focus of armed attacks over the past year.
US forces believe armed groups have been flowing into Iraq across the border from Syria.
Many of the attacks in and around Mosul in the past have been claimed by al-Qaida in Iraq, a group headed by Jordanian Abu Musab al-Zarqawi and allied to Osama bin Laden's al-Qaida network.
Later in the day, three people were killed and 17 others injured when a car bomb exploded near a group of Iraqis drinking alcohol on the banks of the Tigris river in Baghdad, the Interior Ministry reported.
The vehicle exploded near a bridge in the Sunni Muslim Adhamiyah district of the capital. An AFP journalist on the scene saw three cars ablaze.
In another attack, Faisal Khazali, the shaikh of the Khazali tribe, was shot dead when gunmen opened fire as he drove through the Al-Alam neighbourhood in southwestern Baghdad, the Interior Ministry said.