A car bomb at the entrance to an Iraqi army base while new recruits were massed at the gate killed at least 25 people on Tuesday, in the country's deadliest single attack in more than three months.

The attack was not owned by any group, but previous bombings against security force recruitment drives have been claimed by Al-Qaeda's front group in Iraq, which views soldiers, policemen and civil servants as supporters of the Baghdad government.

The midday (0900 GMT) blast in the town of Taji, 25 kilometres (15 miles) north of the capital, is likely to raise fresh concerns about the capabilities of Iraq's security forces 11 months after the departure of American troops.

A medic put the toll at 25 dead and 40 wounded from the car bomb, while an interior ministry official said 26 people were killed and 30 were hurt. It was not immediately clear how many of the victims were soldiers or new recruits.

Differing tolls and breakdowns of casualty figures are common in the chaotic aftermath of violence in Iraq, which is typically carried out by Sunni militants bent on targeting the Shiite-led government and its institutions.

Heavy security was placed around Kadhimiyah hospital where many of the victims were being sent, an AFP journalist said, with family members of recruits rushing to the facility for news of their relatives.

Journalists were barred from taking photographs or speaking to victims.

"I was home when the bomb went off," said a man who identified himself only as Jawad, whose son was wounded in the blast. "But when it exploded, I went straight to the hospital, because I knew my son was at the base, and I expected he would be hurt. I was right."

The explosion struck as the base was holding a recruitment day to welcome potential new soldiers, events which have previously been targets of militants intent on carrying out mass-casualty attacks in Iraq.

On January 18, 2011, a suicide bombing in the middle of a crowd of police recruits in Tikrit, north of Baghdad, killed 50 people and wounded 150, and on August 17, 2010, another suicide attacker killed 59 army recruits and wounded 125 others.

Officials insist Iraq's security forces are capable of largely maintaining internal stability, despite regular deadly attacks nationwide, but they are widely acknowledged to be unable to protect the country's borders, airspace or maritime territory.

Their numbers have been stretched, however, as increasing numbers of soldiers have been deployed to Iraq's border amid fears the violence ravaging Syria could spill across the frontier.

The car bomb was the second blast in as many days in Taji -- an explosion in the confessionally-mixed town on Monday wounded seven people.

Tuesday's bombing was the deadliest single attack to hit Iraq since July 23 when a series of coordinated blasts, also in Taji, killed 42 people on what was the country's deadliest day in two and a half years.

The latest deaths came after figures released on Thursday showed a sharp decline in attacks last month, with the number of people killed the lowest since June.

Violence is sharply lower nationwide since it peaked during Iraq's brutal sectarian war in 2006 and 2007, but attacks remain common, especially in and around Baghdad.