On Sunday, Pope Francis sent prayers to the victims of the concert hall attack in Moscow, describing the violence as "inhuman acts" that offend God.
"I assure my prayers for the victims of the cowardly terrorist attack carried out the other evening in Moscow," said the 87-year-old pontiff after a Palm Sunday mass in Saint Peter's Square.
"May the Lord welcome them in his peace and comfort their families, and convert the hearts of those... who organise and carry out these inhuman actions which offend God, who commanded 'Thou shalt not kill'."
The pontiff presided over Palm Sunday Mass in front of an estimated 25,000 people, but spoke in a weak, shaky voice and did not deliver his homily as scheduled.
He appeared in good spirits following mass, however, circling the piazza, smiling and waving at the crowd from his vehicle.
The pontiff caught flu last month and has subsequently asked others to read his texts at appearances.
Francis has suffered a number of health issues in recent years, from knee and hip pain to an inflamed colon and hernia surgery last year.
Sunday's liturgical celebration marked the start of Holy Week, in which the pope presides over a series of important events leading up to Easter, Christianity's most important holiday.