Pope Francis called on the world to give tangible help to persecuted Christians on Monday, highlighting the plight of people he described as modern "martyrs" after nearly 150 people were killed by Islamist militants in Kenya. 

Addressing crowds in bright sunshine at St. Peter's Square in the Vatican after four days of special Easter services, the pontiff listed the evils inflicted on people for their faith and exhorted the international community to act.

"These are our martyrs of today, and they are many. We can say that there are more of them now than there were in the early centuries. I hope the international community does not look on, mute and inert, at such an unacceptable crime," Francis said.

In Kenya on Thursday, gunmen from the al Qaeda-aligned group al Shabaab targeted Christians for execution after storming the Garissa University College campus, some 200 km (120 miles) from the Somali border.

Kenyan churches have responded to the attack by hiring armed guards, as the massacre resonated throughout the Catholic Church's weekend commemoration of Jesus's crucifixion.

Francis has expressed alarm about the targeting of Christians for their faith, and decried incidents including the decapitation of 21 Egyptian Copts in Libya in February.

During Monday's address, he called for "concrete participation and tangible help to defend and protect our brothers and sisters who are persecuted, exiled, killed, beheaded, solely because they are Christians".

The leader of the world's 1.2 billion Catholics has said the international community would be justified in using military force as a last resort to stop "unjust aggression" perpetrated by Islamic State militants.

The Vatican's official preacher, Father Raniero Cantalamessa, said on Friday there was a "disturbing indifference" among world institutions and in public opinion to the killing of Christians, mentioning the bloodshed in Kenya and Libya.