VATICAN CITY (CNS) -- For at least the fourth time since Russia began its large-scale attack on Ukraine in February 2022, Pope Francis and Ukrainian President Volodymyr Zelenskyy spoke by telephone Dec. 28.

In his daily video report posted on X, Zelenskyy said, "I just had a call with Pope Francis."

 "I expressed my gratitude to His Holiness for his Christmas greetings to Ukraine and Ukrainians, as well as his wishes for a just peace for all of us," the president continued in the video, which was subtitled in English. "We discussed our joint work to put Ukraine's Peace Formula into action. Over 80 countries are already involved in this process at the level of their representatives. And there will be more of them."

The "peace formula" is a set of conditions Zelenskyy has insisted are key to ending the war; they include the release of prisoners and the return of children forcibly taken to Russia; the withdrawal of Russian troops and restoration of Ukraine's territorial integrity; food security; the prosecution of war crimes; and the remediation of ecological damage.

The Ukraine Peace Forum is an effort to rally global support for the peace formula.

 "I am grateful to the Holy See for supporting our efforts," Zelenskyy said in the video.

Pope Francis has prayed for peace and urged others to do so in almost every public address he has made since the Russian aggression began.

In his address Christmas Day, he said: "Contemplating the baby Jesus, I implore peace for Ukraine. Let us renew our spiritual and human closeness to its embattled people, so that through the support of each of us, they may feel the concrete reality of God's love."

Two days after the Russian invasion, Pope Francis and President Zelenskyy spoke on the phone for the first time, they had another phone conversation less than a month later and again in August 2022.

The pope and president met in person at the Vatican in May, and a week later the Vatican confirmed that Pope Francis had appointed Italian Cardinal Matteo Zuppi of Bologna as his envoy "to help ease tensions in the conflict in Ukraine."

From June to September, the cardinal visited Moscow, Kyiv, Washington and Beijing trying to promote peace, ensure the safety of grain exports from Ukraine and pressure Russia to return the thousands of children Ukraine and human rights organizations have said have been taken to Russia.