Pope Francis on Wednesday issued a fresh appeal for peace in Gaza and Ukraine, deploring the killing of aid workers in an Israeli airstrike and paying tribute to a Ukrainian soldier who died in the war against Russia.

Seven people working for the aid charity World Central Kitchen (WCK) were killed on Monday in Gaza in what Israeli Prime Minister Benjamin Netanyahu has called an unintended and "tragic" incident.

"I express deep regret for the volunteers killed while they were engaged in distributing humanitarian aid in Gaza. I pray for them and their families," Francis said during his weekly audience.

The 87-year-old pontiff has been in poor health in recent weeks, limiting his public speaking or cancelling some engagements during Easter week, but on Wednesday he took part in full in the outdoor audience.

He renewed calls for an "immediate" Gaza ceasefire, the release of all Israeli hostages kidnapped by Palestinian militants Hamas, full access for humanitarian aid, and warned against any "irresponsible" regional widening of the conflict.

Turning to "martyred" Ukraine, Francis told crowds in St Peter's Square that he was holding in his hands a copy of the New Testament and a set of rosary beads belonging to a slain 23-year-old Ukrainian soldier.

"I would like us all, in this moment, to have a bit of silence, thinking about this young man and many others like him who have died in this folly of war. War always destroys, let's think about them and pray," he said.

Francis said the soldier, who he identified only as Oleksandr, died in Avdiivka, an eastern Ukrainian city captured by Russians in February.

He had already mentioned his belongings at another audience last month, after they were given to him by a nun who had been on several charity missions to Ukraine.