COLOMBO (AFP) - Sri Lanka on Monday admitted for the first time that civilians may have been killed during its military push against Tamil Tigers rebels in 2009 that has been dogged by allegations of war crimes. A defence ministry report entitled "Humanitarian operation factual analysis" said they followed a "zero civilian casualty policy" but in the face of a formidable enemy it was impossible to fully implement it. "Despite clear intent of the government of Sri Lanka and the numerous precautions taken, it was impossible in a battle of this magnitude, against a ruthless opponent actively endangering civilians, for civilian casualties to be avoided," the 161-page report said. President Mahinda Rajapakse, who is also the commander-in-chief, marked the first anniversary of crushing the Tigers last year insisting that not a single civilian was killed by his troops. "Our troops carried a gun in one hand and a copy of the human rights charter in the other," Rajapakse said at an anniversary victory parade attended by thousands of troops in Colombo in June last year. "Our guns were not fired at a single civilian." He has also insisted that he will not allow any of his troops to be taken before international war crimes tribunals. Senior military officials had privately said there may have been civilian casualties while crushing the separatist Liberation Tigers of Tamil Eelam (LTTE), but Monday's report is the first acknowledgement of it in an official document. The report gave no estimates for the number of civilian casualties. The United Nations and rights groups have said that the number could be as high as 40,000. Defence Secretary Gotabhaya Rajapakse, the president's younger brother who unveiled the official report in Colombo, said claims of high civilian casualties were designed to tarnish the image of the country. "Take for example this accusation of 'up to 40,000 civilian casualties' occurring during the humanitarian operation," he said. "This is a vague accusation, based on even vaguer arithmetic, which keeps getting repeated without any sort of critical analysis by people who should know better."