KABUL - Afghan police and coalition forces killed 31 militants in a Taliban controlled region of the country's opium poppy-growing belt, the second large battle in the Afghan south in two days, officials said Wednesday.

The battle in three villages in the Kajaki region of Helmand province on Tuesday also wounded 20 militants, the Interior Ministry said.

Kajaki is the site of a U.S.-funded dam that provides hydroelectric power to much of southern Afghanistan. While a small unit of British troops protects and controls the dam, those forces are surrounded by hostile militants on all sides.

The Afghan government admits it has little control in that region of Helmand. U.S. President Barack Obama is sending 21,000 additional U.S. troops to Afghanistan this year, and many thousands are expected to deploy to Helmand to aid the British and other international forces who have faced pitched battles in the dangerous province.

Helmand is the world's largest opium poppy-growing region, and United Nations officials estimate the Taliban and other drug lords derive up to US$500 million a year from the illegal trade.

News of the battle in Kajaki came one day after a police chief in Uruzgan province said Afghan and foreign troops killed 30 Taliban fighters in his province.

Violence in Afghanistan is expected to surge this year as the new U.S. troops arrive. Militant attacks have grown increasingly deadly the last three years, and insurgents now control wide swaths of countryside where Afghan and international forces don't have enough manpower to maintain a permanent presence.

Canada currently has more than 2,500 troops serving in Afghanistan, most of whom are stationed in Kandahar Province in the southern part of the country.