A donkey cart carrying sand was left unattended in the market area of Mastak town near the Afghan border, officials said.

"The explosives concealed in the sand loaded onto the cart was detonated with the help of a remote controlled device," local administration official Syed Ahmed Jan told AFP by telephone.

Another senior official Mutahir Zaib confirmed the incident and the casualties. He said the injured were out of danger.

No group has claimed responsibility. Officials have blamed previous such attacks on Taliban and Islamist militants.

At least 12 people were killed in two separate militant attacks in the troubled Khyber district on Saturday.

Some 18,000 people fled their homes in Khyber last month amid fears of a fresh onslaught of fighting between the army and Islamist militants tied to the Pakistani Taliban.

Pakistan's army has previously launched a series of offensives targeting the the Lashkar-e-Islam (LI), a Taliban-allied militant group waging a local insurgency.

Pakistan's seven tribal districts near the Afghan border are rife with a homegrown insurgency and are strongholds of Taliban and Al-Qaeda operatives.