Violence erupted on Monday when people of one clan tried to divert water from an irrigation canal to their fields and men from another clan resisted.

The rivals battled with rifles, mortars and rocket-propelled grenades, said the region's administrator, Mohammad Salim Khan.

The government said 14 people had been killed and 35 wounded. A military official who declined to be identified said 22 people had been killed.

"There's a lull now and the tribesmen have started withdrawing from their positions on the mountains," Khan told Reuters.

The ethnic Pashtun communities that lie on both sides of the Pakistani-Afghan border have been traditionally well-armed.

Pakistan has about one fifth less water this year than normal, said an official at the Indus River System Authority, and several crops including rice have been hit hard.

But shortages are only expected to get worse as populations and demand grow. Analysts say water disputes will become more common, not only within communities but between countries.