MINGORA, Pakistan (Reuters) - Thousands of people are fleeing a valley in north-western Pakistan after security forces warned them to leave ahead of a major operation against pro-Taliban militants, witnesses and officials said on Tuesday.

Helicopter gunships and artillery killed two militants and wounded around 15 when troops targeted their vehicle near the town of Alpuri in the scenic Swat valley's Shangla district, residents said. Eighteen insurgents were killed on Monday.

Four civilians were killed when a mortar bomb hit their residence as security forces pounded militant positions in the same district, witnesses said. The military has said 15,000 troops will soon launch a major offensive in Swat.

"In certain areas the local population was advised to leave their homes to avoid collateral damage," Interior Ministry spokesman Javed Iqbal Cheema told reporters in Islamabad.

Cheema said some 130-140 militants had been killed over the last week, taking the death toll in Swat to nearly 300 since late October. That was when fighting flared between troops and fighters of a pro-Taliban cleric, Maulana Fazlullah, who is leading a campaign to enforce a strict Islamic code in the area.

Militants have infiltrated in from strongholds on the Afghan border to support Fazlullah in recent weeks, while the country's focus has been on President Pervez Musharraf's declaration of emergency rule and suspension of the constitution in an apparent bid to hold onto power.

The army launched a fresh push last week to wipe out militant positions after police failed to prevent a build-up in the valley in North West Frontier Province.

The military says militants are using civilian premises to launch attacks on security forces.

Witnesses said residents were leaving the area in large numbers after security forces broadcast warnings by loudspeaker.

"We have no option but to leave our home," said Ali Ravi, who lost three members of his family on Sunday night when a mortar bomb hit his residence in Tottvandi village in Kabal.

Swat, close to Pakistan's lawless tribal belt bordering Afghanistan, has seen a surge in militant activity since Fazlullah launched an illegal radio station and urged a holy war after an army assault on a militant mosque in Islamabad in July.

Northwest Pakistan has also seen a upswing in sectarian violence, with more than 100 people killed in fighting between Sunni and Shi'ite Muslim tribesmen in the Kurram tribal region near the Afghan border over the past week.

Sectarian violence has bedevilled Pakistan since the 1980s and Kurram has a long history of such clashes.