Pope Benedict said in an annual peace message on Tuesday that countries have a duty to respect international humanitarian law even if they are at war.
In the first peace message of his pontificate, he also appealed for world-wide nuclear disarmament and said countries considering acquiring such weapons should "change their course".
In the 12-page message, called "In Truth, Peace", he said the Holy See was convinced international humanitarian law had to be respected "even in the midst of war".
The Pope did not name any countries or wars but his words followed controversy over reports of abuse of prisoners by the United States in Iraq and at Guantanamo Bay. In another part of the message, the Pope said the possession of nuclear weapons by any country for security was "not only baneful but also completely fallacious" because there would be no winners in a nuclear war.