VATICAN CITY (Reuters) -Pope Francis has been forced to cancel a trip to Florence this Sunday and Ash Wednesday services next week because of an acute flare up of pain in his knee, the Vatican said on Friday.

       The 85-year-old pope suffers from sciatica, a nerve condition that causes pain in his legs. He has recently read a few speeches sitting down, citing pain in his knee.

       Vatican spokesman Matteo Bruni said the pope's doctor had ordered "a greater period of rest", without elaborating.

       In Florence, Francis was due to say a Mass to conclude a meeting of bishops and mayors from the Mediterranean area. The Mass would have required him standing for a few hours.

       On Ash Wednesday, he was due to lead a procession between two Rome churches on the first day of the Christian season of Lent.

       On Thursday night, Francis took part in an online conversation with university students from around the world. He sat behind a table in a room in his residence and did not appear to be in any pain, conversing and joking with the students.

       On Friday morning he went unannounced to the Russian embassy to the Holy See near the Vatican to relay his concern over Russia's invasion of Ukraine to Moscow's ambassador. [L1N2V00WS]

       Francis was hospitalised for 11 days in July for surgery to remove part of his colon. After a period of rest he made a grueling trip to Hungary and Slovakia in September.

       He has been in general good health since but has occasionally complained of pain in his leg and knee.

       (Writing by Philip Pullella. Editing by Crispian Balmer and Keith Weir)