Religion should not be used as a prop for power, Pope Francis told an inter-faith meeting in ex-Soviet Kazakhstan on Wednesday.

"May we never justify violence. May we never allow the sacred to be exploited by the profane. The sacred must never be a prop for power, nor power a prop for the sacred," he said.

Francis spoke a day after appealing for peace and an end to the "senseless and tragic war" in Ukraine.

A simultaneous visit to the country by Chinese leader Xi Jinping has led to speculation of a possible historic meeting.

"Mindful of the wrongs and errors of the past, let us unite our efforts to ensure that the Almighty will never again be held hostage to the human thirst for power," Francis said.

"Let us free ourselves of those reductive and destructive notions that offend the name of God by harshness, extremism and forms of fundamentalism, and profane it through hatred, fanaticism, and terrorism," he said.

"Let us commit ourselves, then, even more to insisting on the need for resolving conflicts not by the inconclusive means of power, with arms and threats, but by the only means blessed by heaven and worthy of man: encounter, dialogue and patient negotiations," he said.

The 85-year-old Argentine pope, who is forced by knee pain to use a wheelchair and has admitted he must slow down or consider retirement, is on his 38th trip abroad since becoming pontiff in 2013.

Russian Orthodox Patriarch Kirill, a close ally of President Vladimir Putin, was initially expected but has pulled out of the September 14-15 event, dashing hopes of a meeting with Francis over the Ukraine conflict.

While the pope has called for peace and denounced a "cruel and senseless war", Kirill has defended Putin's "military operation" and the fight against Russia's "external and internal enemies".

About 100 delegations from 50 countries are expected to take part in the event in Kazakhstan, a former Soviet republic that gained independence in 1991.

Francis is the second pope to visit Kazakhstan after John Paul II's trip in September 2001.

He said last week that doctors had forbidden him from travelling to Ukraine or Moscow for now, as he recovers from a knee problem that has forced him to cancel numerous events at the Vatican.