Pope Francis has called for authorities to do more to secure the release of an Italian priest and two Orthodox bishops kidnapped by extremists in Syria.

The Greek Orthdox bishop of Aleppo, Boulos Yazigi, and the Syriac Orthodox bishop of the same city, Gregorios Yohanna Ibrahim, were both kidnapped in early 2013 when they tried to secure the release of two kidnapped priests. It's reported their car was ambushed on the way to the negotiations.

Months later on July 29 2013, Jesuit priest Fr Paolo Dall'Oglio was kidnapped. He had worked in Syria for more than 30 years, and refused to leave when the country's civil war began in 2011. Around 250,000 have died in the conflict so far, with many kidnapped.

Pope Francis told crowds in St Peter's Square he wanted "a renewed commitment" from Syrian and international authorities to secure the release of the three clergymen, "so these brothers of ours can regain their freedom soon".

The pontiff also became the first person to sign up for the Catholic Church's annual World Youth Day next year, which will be held in Krakow, Poland.

He subsequently opened up registration for young people around the world to sign up for the 2016 gathering.