VATICAN CITY (Reuters) - Pope Francis expressed deep sadness at the beheading of 21 Egyptian Coptic Christians in Libya, departing from the script of an address on Monday to emphasise the unity of all Christians regardless of the denomination they follow.

Addressing members of the Church of Scotland, the Argentine pope mentioned the killings which took place on a beach in Libya and were filmed and broadcast on Sunday by a website that supports Islamic State militants.

"Their only words were: 'Jesus, help me!' They were killed simply for the fact that they were Christians," Francis said in his native Spanish, departing from the Italian he uses at most formal events.

The leader of the world's 1.2 billion Catholics, who has said it is "lawful" to stop an unjust aggressor, went on:

"The blood of our Christian brothers and sisters is a testimony which cries out to be heard. It makes no difference whether they be Catholics, Orthodox, Copts or Protestants. They are Christians!"

Francis added: "The martyrs belong to all Christians."

Later on Monday, the Vatican's spokesman said Francis had telephoned the Coptic pope, Tawadros II, to say he was sharing the pain caused by the "barbaric murder of Christian Copts by Islamic fundamentalists".

Egypt, where Christians make up around a tenth of a predominantly Muslim population, is stepping up its battle against Islamist militants in neighbouring Libya.

The Coptic Church is founded on the teachings of St. Mark, who took Christianity to Egypt in the first century during the reign of the Roman emperor Nero, according to tradition.

(Reporting by Isla Binnie; Editing by Mark Heinrich)