ISLAMABAD, Pakistan — Senior Pakistani officials said yesterday that they believed Islamic militants were responsible for a suicide bombing at an army training base that killed at least 42 soldiers and injured about 20 others.

If those suspicions are borne out, it would be the deadliest attack mounted by pro-Taliban insurgents against Pakistan’s military, which is allied with America in the hunt for figures linked to Al Qaeda.

No group issued a claim of responsibility, but militants had vowed to avenge a military raid on an Islamic religious seminary last month that killed about 80 people.

Pakistan’s government said most of the dead in the October 30 helicopter attack were militants who were receiving training; authorities at the religious school, or “madrassa,” said the majority were students with no ties to any radical group.

The scene of yesterday’s suicide bombing was a base in Dargai, about 85 miles northwest of Islamabad in the restive Northwest Frontier Province, which borders Afghanistan.

The army said in a statement that a lone attacker, wrapped in a flowing cloak, rushed into an unfenced training area and blew himself up amid several hundred soldiers who were just beginning their morning calisthenics. Many were new recruits in their teens.

Speaking to reporters in Islamabad, Information Minister Mohammed Durrani called the suicide attack a “contemptible act.”

Troops launched a manhunt for suspected accomplices, the army said, but no arrests were immediately reported.

Officials said Dargai is known to be a center of activity for the banned Islamic group Tehrik-e-Nifaz-e-Shariat Mohammadi, or the Movement for the Implementation of Islamic Law, whose members are dispersed throughout the tribal regions bordering Afghanistan.

One of the group’s senior leaders was killed in the “madrassa” raid in the Bajur tribal region, drawing pledges of retaliation from surviving leaders. Pakistani officials said intelligence indicated the group had made good on the threat.

Interior Minister Aftab Khan Sherpao said of the suicide attack on the base: “It is linked to the Bajur incident.” The banned group, which seeks to impose a Taliban-style social system, sent many of its adherents into Afghanistan to fight American troops in the aftermath of September 11, 2001.

President Musharraf of Pakistan condemned the suicide attack at the base and promised to “deal with it very forcefully,” the Information Ministry said.

Analysts said the attack on the base could represent a worrying change in tactics by anti-government militants. Unlike in Afghanistan, suicide bombings have been much less frequently employed in Pakistan.

“It is a new phenomenon here and previously was used for sectarian violence,” an analyst at Lahore University, Rasul Baksh Raees, said.