ISLAMABAD, Pakistan -- Eight tribesmen allied with the government against al-Qaida-linked foreign fighters were killed in northwest Pakistan, authorities said Monday.

No one took responsibility for the attacks in the South Waziristan region. But those killed were loyal to pro-government tribal leader Maulvi Nazir, a rival of local Taliban leader Baitullah Mehsud. The government has accused Mehsud of involvement in the assassination of opposition leader Benazir Bhutto, as well as a string of other attacks.

The army said the eight were killed when militants fired rocket in two separate attacks Sunday night on offices of Nazir's supporters. Local intelligence officials said three tribesmen were killed in the town of Wana, and a second strike in the nearby Shakai village killed five tribesmen and wounded six. The officials spoke on condition of anonymity because they were not authorized to talk to journalists.

An escalation in tensions in South Waziristan could spark a surge of violence in a region laden with weaponry where the state has scant control. Al-Qaida leader Osama bin Laden and his top deputy Ayman al-Zawahri are believed to be hiding somewhere in Pakistan's lawless tribal regions bordering Afghanistan.

Pakistan, a key U.S. ally in its war on terrorism, is under growing pressure to contain the wave of militancy sweeping the border regions, particularly South Waziristan, which is also a staging point for attacks on U.S. and NATO forces in Afghanistan.

Nazir led Wazir tribesmen in a government-backed campaign last year to push al-Qaida-linked foreign militants out of South Waziristan. The foreign militants, mostly Uzbeks, were banished from the surrounding Wana Valley after the fighting in March and April, which left scores dead.

After the killing of the eight tribesmen, Nazir's spokesman ordered people from the rival Mehsud tribe to leave the Wana Valley by 8 a.m. today.

"If they pay no heed to our warning and remain in our area, they [the Mehsuds] would be responsible for any harm caused," spokesman Lal Wazir told reporters in Wana.

Intelligence officials said Nazir's men killed one Mehsud tribesman Monday and abducted four in Wana in retaliation for the deadly rocket attacks.

Residents said tensions have been growing in South Waziristan since Mehsud last month formed an umbrella group for various pro-Taliban forces fighting in the tribal regions.

Nazir has refused to join Mehsud's coalition, the residents said, but he has acknowledged that he fought for the Taliban when the group's hard-line regime was in power in Afghanistan.

Many of the Uzbek fighters Mehsud has harbored fled Afghanistan after the collapse of the Taliban regime in a U.S.-led invasion in late 2001.

The government claimed as many as 230 Uzbeks were killed in last year's clashes, although many observers doubted that many died. It was touted as a success for Pakistan's policy to get tribesmen -- rather than security forces -- to police the Afghan border regions.

The government has blamed Mehsud for a series of suicide attacks on security forces, top government officials and Bhutto. Her Dec. 27 assassination sparked rioting by her supporters and deepened the political turmoil in Pakistan -- prompting a six-week delay in parliamentary elections.

Mehsud has denied involvement, and Bhutto's supporters accuse the government of playing a role in the attack, which it denies.

On Sunday, Pakistan reiterated that it will not let American forces hunt al-Qaida and Taliban militants on its soil, after The New York Times reported that the Bush administration was considering expanding U.S. military and intelligence operations into Pakistan's tribal regions.

The government of President Pervez Musharraf views it as a breach of sovereignty that would further inflame anti-American sentiment in Pakistan.

Elswhere in northwestern Pakistan on Monday, a suicide attacker driving a pickup truck detonated a bomb near a security post, wounding eight soldiers and two civilians, the military said in a statement.

The attacker died in the blast, which happened in Kabal in the Swat region, a former tourist destination where security forces have been battling loyalists of a pro-Taliban cleric. Swat is about 175 miles north of South Waziristan.