Pope Benedict has called on countries to respect international humanitarian law even if they are at war and appealed for worldwide nuclear disarmament.
In the first peace message he said countries considering acquiring such nuclear should "change their course".
In the message for the Church's World Day of Peace, celebrated on January 1st, he also strongly condemned terrorism but said the world community should look deeper into its political, social, cultural, religious and ideological motivations.
In one part of the message, which is sent to heads of state and international organisations, the Pope said war could not be an excuse for disregarding international humanitarian law.
"The truth of peace must also let its beneficial light shine even amid the tragedy of war," he said, reinforcing his stand by quoting from another Vatican document that said "not everything automatically becomes permissible between hostile parties once war has regrettably commenced".
In the 12-page message, called In Truth, Peace, he said the Holy See was convinced that international humanitarian law had to be respected "even in the midst of war".
He referred to international humanitarian law as one of the finest expressions of truth. "Precisely for this reason, respect for that law must be considered binding on all peoples," he said.
International humanitarian law "must be brought up to date by precise norms applicable to the changing scenarios of today's armed conflicts and the use of ever newer and more sophisticated weapons", he added.
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.
The Pope did not name any countries or wars but his words followed widespread controversy over reports of abuse of prisoners by the United States in Iraq and at Guantanamo Bay.