MINGORA, Pakistan: Thousands of people fled from a valley in northwest Pakistan as security forces stepped up an offensive against pro-Taliban militants, while fighting killed 19 people. Advancing ground troops killed 15 militants in the Shangla district in the scenic Swat valley, the site of fierce clashes with insurgents led by hardline Islamist cleric Maulana Fazlullah in recent weeks in which more than 300 people have died.

Four civilians were killed when a mortar bomb hit their residence as security forces pounded militant positions.

The military has moved 15,000 troops to the area for the offensive.

A military spokesman said 30 militants were killed in fighting in Swat on Monday.

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 the Afghan border, 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.