Pope Leo lamented a world "in flames" due to wars and the destruction of the environment during an Ash Wednesday Mass, opening the season of Lent for the world's Christians.

Before sprinkling ashes on the heads of participants, a sign of mortality, the pope said the ashes could represent "the weight of a world that is ablaze, of entire cities destroyed by war."

He also told participants the ashes could signify "the ashes of international law and justice among peoples, (and) the ashes of entire ecosystems."

"It is so easy to feel powerless in the face of a world that is in flames," said Leo, the first US pope.

Lent is a 40-day period of penance that leads to Easter, the most important Christian holiday, which celebrates the day on which believers say Jesus rose from the dead.

It represents the 40 days Jesus is said in the Bible to have spent fasting in the desert. During the season, Catholics are asked to fast, remember the needy and reflect on mortality.

Leo, elected leader of the 1.4-billion-member Church in May to replace the late Pope Francis, did not mention a specific conflict in his remarks.

The pope has forcefully decried the world's ongoing wars in his first year and denounced what he called a global "zeal for war" in a major foreign policy speech last month.

He spoke on Wednesday at a service held in the Basilica of Santa Sabina on Rome's Aventine Hill, preceded by prayers in a nearby church and a procession of cardinals and bishops.