Sri Lanka has acknowledged for the first time that civilians may have been killed in the last phase of the country's quarter-century-long civil war against the Tamil Tiger rebels.

A defense ministry report released Monday said although the military followed a “zero civilian casualty policy,” it was impossible to avoid such casualties given the magnitude of the fighting and the “ruthlessness” of the opponent.

Sri Lanka's civil war ended in 2009 with the military's defeat of the Tamil Tigers. A United Nations panel has estimated that thousands of civilians were killed in the final phase of the conflict. Earlier this year, the panel said it had found credible allegations of human rights violations, including possible war crimes, committed by both the military and the rebels.

The government denied the allegations.

In its report, the Sri Lankan defense ministry did not give an estimate of the number of civilians killed.

Defense Secretary Gotabhaya Rajapaksa on Monday rejected claims by some rights groups that as many as 40,000 civilians were killed in the final months of the war. The defense secretary, who is Sri Lankan President Mahinda Rajapaksa's brother, called the figure a “vague accusation based on a vague arithmetic” meant to tarnish the image of the country.