Armed forces must only be used as legitimate defense, never to dominate another country, a breathless Pope Francis said Sunday, presiding over an outdoor mass despite a bout of bronchitis.

"This armed service is to be exercised only in self-defense, never to impose domination over other nations, (and) always observing international conventions on conflict", Francis said.

He told military forces gathered for a special Armed Forces Mass in St Peter's Square that they should consider themselves "servants of the security and freedom of their peoples."

"Let us pray for peace.... Let the guns be silent everywhere and let the cry of the peoples asking for peace be heard," Francis added at the end of the Angelus prayer.

During the mass earlier Sunday, the 88-year-old head of the Catholic Church asked an aide to finish reading his homily, blaming what he described as "breathing difficulties."

The pope was sitting on a raised platform in St Peter's Square, his garments whipped by the wind, which at a one point even tore off his white papal "zucchetto", or skull cap.

Francis, who has seemed breathless for the past week, held his meetings at home in the Vatican on Friday and Saturday so he could rest, the Holy See said.

The Argentine pontiff has been plagued in recent years by health issues, from knee and hip pain to an inflamed colon. He also underwent surgery on a hernia.

The pope, who had part of one of his lungs removed as a young man, was admitted to hospital for three nights in 2023 with bronchitis, which was cured with antibiotics.