ISLAMABAD, Pakistan — Pakistan’s military says it achieved a major strategic victory over Islamist militants hiding in the Shawal Valley, a thickly forested area bordering Afghanistan thought to be among the last few refuges here for al-Qaida and the Pakistani Taliban.

Pakistan’s army chief, Raheel Sharif, visited the area on Friday to congratulate troops for clearing “all peaks” that surround the valley. Now, Sharif said, the army will began a final assault on the lower elevations.

“We will not stop unless we achieve our end objective of a terror-free Pakistan,” Sharif said in a statement.

After years of bloodshed from terrorist attacks, the Pakistani military launched its offensive in the country’s northwestern tribal areas last June. Since then, army leaders say they have driven militants from much of North Waziristan, which had been a safe-haven for terrorist groups that carried out attacks both in Pakistan and Afghanistan.

Pakistan’s interior minister, Chaudhry Nisar Ali Khan, said Saturday that 20,000 militants had been killed or wounded since the operation began. Another 2,500 have been arrested, he said.

Pakistan restricts media access to the tribal areas, which makes it difficult for independent verification.

But Pakistan has experienced a sharp decline in civilian casualties from terrorist attacks.

Many of the militants from North Waziristan fled across the border into Afghanistan, where violence has been increasing. Others are hiding along the immediate Pakistan-Afghanistan border, officials said.

The Shawal Valley straddles both North and South Waziristan, but it also includes trails and tunnels to Afghanistan.

U.S. drone strikes frequently target the area. In January, a U.S. drone strike mistakenly killed two foreigners who had been kidnapped by al-Qaida, including American Warren Weinstein.

Zahid Hussain, an Islamabad-based military analyst, said the Pakistani army is trying to complete its offensive in the Shawal Valley before snow begins falling there in September.

“This is going to be the most critical phase of the North Waziristan operation,” said Hussain, who has made several visits to the area. “It’s a very, very treacherous, mountainous area, and thickly forested, so there is a reason why the army has left it for the end.”