Streams of civilians jammed into cars and trucks to flee the militant stronghold of South Waziristan as the government pounded the area with air strikes ahead of an expected ground offensive against the Taliban along Pakistan's border with Afghanistan.

Early Thursday, a suspected U.S. missile strike hit neighboring North Waziristan, killing four alleged militants as the Americans, too, kept up pressure on insurgents in Pakistan's lawless tribal belt, intelligence officials said. Separately, an explosion outside a police station rocked a district near the tribal regions, killing at least 10 people.

Bombing runs over suspected militant hide-outs have sharply increased in recent days after a string of bloody attacks on military and civilian targets killed scores of people across Pakistan. Government officials said the wave of terror was forcing them to take the fight to the insurgents' heartland.

The army, which gave no timeframe for the offensive, has reportedly already sent two divisions totaling 28,000 men and blockaded the region.

Fearing the looming offensive, about 200,000 people have fled South Waziristan since August, moving in with relatives or renting homes in the Tank and Dera Ismail Khan areas, a local government official said, speaking on condition of anonymity because he was not authorized to talk to the media.

The exodus intensified with the increase in airstrikes in recent days.

Since the weekend, about 80 vehicles a day have been carrying fleeing families past one checkpoint at Chonda on the edge of Dera Ismail Khan, said Naimatullah Khan, a police officer based there.

Police at the isolated checkpoint stopped the departing vehicles Wednesday, checked people's identification, searched their luggage and frisked some of them. Many of those fleeing had to take circuitous routes over back roads to dodge the military blockade.

Haji Ayub Mehsud, 55, said the increased bombing forced him to flee with his six children.

"It is difficult for local people to stay there in peace. I had to bring out my family," Mehsud told an AP reporter at Chonda.

In new bombing Wednesday afternoon, military jets pounded a cave in the Spinkai area, killing eight people, officials said.

Local tribesmen said the victims were all civilians, including three women and three children, who had abandoned their home and fled to the cave to seek shelter from the heavy shelling. The two tribesmen spoke on condition of anonymity because they feared reprisals if they spoke to the media.

Intelligence officials, however, said the bombs hit a suspected militant hide-out, killing eight insurgents. The officials spoke on condition of anonymity because they were not authorized to speak to the media.

Independent confirmation of the attack was not available. The army has barred reporters from the region.

The strike in the rugged, mountainous region came after a wave of air attacks late Tuesday and early Wednesday that killed nine insurgents, the intelligence officials said.

As of last month, at least 80,000 people had registered with the government as displaced from South Waziristan, said Ariane Rummery, spokeswoman for the U.N. refugee agency. Government officials say only half of those fleeing the area have bothered to register.

The U.N. has distributed various types of aid -- from kitchen sets to jerry cans -- to around 6,500 families and is monitoring the situation, Rummery said.

An offensive in South Waziristan is not expected to create anywhere near the refugee problem the government faced when its Swat Valley offensive earlier this year rapidly displaced 2 million people and forced many into tent camps.

The residents of South Waziristan have had plenty of warning about the expected operation, appear to have places to stay nearby and the region has far fewer residents, with population estimates of around 500,000 before the recent exodus.

The U.S. has encouraged Pakistan to take strong action against insurgents who are using its soil as a base for attacks in Afghanistan, where U.S. troops are bogged down in an increasingly difficult war.

The U.S. has carried out a slew of its own missile strikes in South and North Waziristan over the past year, killing several top militants including Pakistani Taliban chief Baitullah Mehsud.

The early Thursday strike hit a compound in Dande Derpa Khel, an area in North Waziristan where members of the militant network led by Jalaluddin Haqqani are believed to operate.

The two intelligence officials who gave word of the strike said the identities of the four killed were unclear, but that they were believed to be militants. The officials spoke on condition of anonymity because they were not authorized to speak to media on the record.

Pakistani formally protests the missile strikes as violations of its sovereignty, but many analysts believe it has a secret deal with the U.S. allowing them.

The Thursday morning explosion occurred in Kohat district, a rough region near the tribal areas. It was not immediately clear what sort of blast it was or whether a suicide attacker was involved. Police official Habib Khan said casualties were feared.