Pope Francis dedicated a portion of his general address last week to the unrest in the Ukraine, and urged the warring sides to push for peace instead of victory.
Thousands have been killed in the war in Donbass, and heavy fighting across the region has increased since January. Francis called on both sides to stop the violence.
"Let us pray to the Lord so that this horrible fratricide violence may cease as soon as possible," he said, hoping a compromise could be reached.
According to the Vatican news service, the Pope admitted that the words "victory" and "defeat" brought him great pain and sadness.
"They are not the right words," he lamented. "The only right word is peace. This is the only right word."
Conflicts between pro-Russian rebels and the Ukrainian government have escalated since the 2014 Ukrainian revolution, and resulted in the self-declared Donetsk and Lugansk People's Republics arming themselves against the government.
Pope Francis grieved the division.
"I think of you, Ukrainian brothers and sisters, this is a war between Christians," he insisted.
Russian Orthodox Church leader Patriarch Kirill I of Moscow thanked the Pope for his call for peace, and for taking a neutral stance in the brutal conflict.
A summit with Western leaders is scheduled for Wednesday, but it is unclear whether it will be postponed as fighting in eastern Ukraine intensified on Tuesday.
Ukrainian President Petro Poroshenko reported that the rebels launched an artillery strike against the town of Kramatorsk, Tuesday morning. Three people were killed and 15 were injured.
