The attack in the region's Ladha sub-district came a day after a similar strike killed at least a dozen militants. It was the fourth suspected US raid in South Waziristan within a week.

Pakistani security forces are preparing for a decisive offensive against Mehsud and his loyalists, who are blamed for nearly 90 percent of terrorist attacks in the country during the last two years.

Limited airstrikes have been carried out by Pakistani jets in recent weeks to soften up Taliban defences, while ground troops have secured land routes ahead of the main onslaught.

Pakistan has protested the US drone attacks, saying they complicated its efforts against militancy by fuelling anti-American sentiments in the country.