The Pakistan army attacked hide-outs of foreign militants in a remote tribal region near Afghanistan on Friday, the third day of clashes that have killed 50 people, the military said.

The skirmishes started Wednesday when Al Qaeda-linked militants fired rockets on army checkpoints near Wana, the main city in Pakistan's South Waziristan (search ) tribal region, sparking an exchange of fire between the troops and rebels.

On Friday, army spokesman Maj. Gen. Shaukat Sultan said that "Today, we appropriately responded to the latest unprovoked attacks by the terrorists."

The Pakistan army later said in a statement that 35 militants and 15 security soldiers had been killed in South Waziristan since Wednesday. It said that the foreign militants had taken the local population hostage, forcing the army to take action to flush them out.

About Friday's assault, it said the army was targeting "known and confirmed hide-outs of miscreants."

Government and security officials said the army was pounding the hide-outs with artillery, mortars and other weapons, while helicopters also were seen flying in the area.

Pakistan's tribal areas bordering Afghanistan are a possible hide-out for Usama bin Laden and his chief aide, Ayman al-Zawahiri (search). There was no immediate indication that top Al Qaeda figures were among those involved in the latest fighting.

Tension has been building in South Waziristan over the past month as authorities have pressured tribesmen to evict hundreds of Central Asian, Arab and Afghan militants, many of whom moved there from Afghanistan after the fall of the Taliban regime in late 2001.

The militants have refused to surrender and register with authorities despite a government amnesty offer that would allow them to settle in Pakistan if they renounce terrorism.

The army and President Gen. Pervez Musharraf (search) - a key U.S. ally in its war on terrorism - recently warned that another military operation could be launched unless the foreign militants give themselves up.