BAGHDAD - Suicide bombers struck in Iraq for a third consecutive day on Wednesday, killing at least 15 people in two attacks in Diyala province and raising fears of a renewed insurgent campaign targeting Iraqi security forces and government officials.

In the deadliest incident, a suicide bomber drove an ambulance packed with explosives into a police training center in the provincial capital, Baqubah, about 35 miles northeast of Baghdad. At least 13 people were killed and 70 injured in the mid-morning attack, many of them trainees with the Facilities Protection Service, which guards state institutions and government dignitaries, according to Lt. Col. Ghalib Attiyah, police spokesman for Diyala province.

In the second attack, Attiyah said, a man wearing an explosives vest blew himself up near the convoy of a top provincial official during a gathering of Shiite pilgrims in the Khalis district, north of Baqubah. The blast killed two and injured 15, including the official, Sadiq al-Hussaini, the deputy chairman of Diyala's provincial council.

It was not immediately clear whether the bomber was targeting the official or the pilgrims, who are frequently attacked by Sunni extremists. Attacks on pilgrims are expected to escalate in the coming days as people set out to participate in commemorations for the Shiite religious holiday of Arbaeen next week.

Wednesday's blasts came a day after a suicide attacker killed 60 people as they lined up to join the police force in Saddam Hussein's home town of Tikrit, shattering more than two months of relative calm with the first major bombing since Prime Minister Nouri al-Maliki formed a new government last month.

The latest bombings brought to four the number of suicide attacks in three days. On Monday, a suicide bomber driving a car killed one person in an attack on the convoy of the governor of the western province of Anbar.

Suicide attacks are generally blamed on the extremist al-Qaeda in Iraq organization, which, although greatly weakened, remains stubbornly present in Sunni areas of central and western Iraq and around the northern city of Mosul.

Overall violence levels are lower than they were a year ago, U.S. officials say, and the Iraqi security forces have claimed major successes against al-Qaeda in Iraq in recent weeks, parading dozens of captured operatives on television.

But the group is also constantly adapting its tactics and will continue to present a major challenge for the foreseeable future, said Gen. Dhiya Hussein Sahi, head of counterterrorism at the Interior Ministry. "They are always changing their strategies, and maybe we will face a new wave of different operations," he said.

Special correspondent Hasan Shammari in Baqubah contributed to this report.