Critics, however, said announcing the deadline makes it easy for terrorists to flee ahead of the operation, as they did when Pakistani forces last month allowed a top al Qaeda terrorist to get away in South Waziristan.
This time, Pakistani forces have shifted their focus to North Waziristan, and more specifically to a group of mud compounds along a forbidding mountain range straddling the Afghan border in the forested area of Shawal.
"There are possibilities of an operation in Shawal," Brig. Mahmood Shah, chief of security for the tribal regions, told The Associated Press on Thursday from the northwestern city of Peshawar. He said some militants in Shawal appear to have escaped the earlier operation, 25 miles to the south. He gave no specifics.
"We have thousands of troops, not hundreds, but I can't give operational details," he said. Shah said intelligence indicated foreign terrorists had used Shawal in the past, and that troops also were concerned about militant activity in two other North Waziristan towns — Shakai and Hamrang, and the village of Makin in South Waziristan.
The government assault in March on al Qaeda suspects holed up in South Waziristan was costly, and failed to net any major terrorists. The military acknowledged it lost at least 50 men, and officials say privately the casualty toll might have been twice that. At least a dozen civilians were killed.
The government says a top al Qaeda militant, the Uzbek terror leader Tahir Yuldash, was injured but managed to escape, possibly through a 1.5-mile-long tunnel that led out of the siege site to a dry riverbed near the frontier. About 160 militants were arrested, and 63 killed. Hundreds escaped.
North and South Waziristan have long been suspected hideouts for al Qaeda leader Osama bin Laden and his No. 2, Ayman al-Zawahri. Senior Pakistani officials initially thought they had al-Zawahri surrounded in March.
On Monday, more than 100 tribal elders met in Peshawar with the local governor, who set the April 20 deadline for turning over the militants to avoid military action. One tribe has formed a 600-member military unit to round up terrorists, though it is not clear how vigorously they will support the military.
Army spokesman Maj. Gen. Shaukat Sultan said Pakistani forces had given tribal elders a small window to cooperate, though he did not say specifically whether the army would wait until the deadline expired before taking action.
"At this moment we are focusing on the political process," Sultan told AP.
Some questioned the government's decision to set a deadline, saying it removed the element of surprise.
"Perhaps those who are handling operations against these terrorists are under the impression that they have encircled them and they have no way to escape," said Talat Masood, a Pakistani military analyst. "It may be true, but I think they are relying on a false presumption."
Shah said military action before the deadline was unlikely, but not out of the question.
"If we have credible intelligence (that terrorists are fleeing), we will go in," he told AP.
Pakistani forces also set a deadline before launching the March operation, and were surprised by the severity of the resistance they faced on a disastrous first day in which at least 16 soldiers died. Entrenched militants used rockets, mortars, grenades and heavy machine-gun fire against a small military unit that was overwhelmed.
The Pakistanis have also been criticized for launching the March assault before U.S. military and Afghan forces in Afghanistan could get in position to catch any terrorists who fled across the frontier.
Lt. Col. Michele DeWerth, a U.S. military spokesman in the Afghan capital, said U.S.-led troops were patrolling the border closely and would continue to conduct "parallel and complimentary" operations on the Afghan side. She declined to give details.
The U.S. ambassador to Afghanistan, Zalmay Khalilzad, caused an uproar this week when he said the American military would move forces into Pakistan if it failed to oust the terrorists itself.
Pakistani President Gen. Pervez Musharraf is a key U.S. ally, but he has refused to allow U.S. military forces to operate on his soil. Khalilzad later backed off the comments.