DERA ISMAIL KHAN, Pakistan — Pakistani jets bombed militant targets in the main insurgent stronghold along the Afghan border today ahead of an expected ground offensive there, while the army killed 26 insurgents elsewhere in the northwest, authorities said.

The army says 80 percent of the militant attacks plaguing nuclear-armed Pakistan are planned from South Waziristan, while the United States says insurgent leaders blamed for spiraling violence in Afghanistan also are based in the lawless, remote area.

The army and the government have agreed to launch what is expected to be a bloody and difficult ground operation in the mountainous region. An army spokesman Monday declined to say when the operation would begin, but there has been speculation it could be imminent.

For the past three months, jets have been bombing targets in the region, and the military has been attempting to cut off militant supply and communication lines. Authorities are also trying to secure the support of militant factions that in the past have agreed not to attack Pakistani troops.

Bombing runs today destroyed about 15 houses in the Makeen, Ladha and Barwand regions of South Waziristan, an official said.

Authorities said helicopter gunship attacks killed 26 insurgents in Bajur, a tribally administered region that is 185 miles north of Waziristan.

The militant threat to Pakistan has been highlighted by four major terrorist attacks over the last nine days, including a suicide attack on a U.N. office in the capital that killed five staffers and a 22-hour siege on the army's headquarters over the weekend.