* Attack is major challenge for new prime minister
* Christians make up about 4 percent of population
* Sectarian violence on the rise amid escalating insurgency
By Fayaz Aziz
PESHAWAR, Pakistan, Sept 22 (Reuters) - A pair of suicide bombers blew themselves up outside a church after Sunday Mass in the Pakistani city of Peshawar, killing at least 40 people in the deadliest sectarian attack since Prime Minister Nawaz Sharif was sworn in in June.
Religious violence and attacks on security forces have been on the rise in past months, undermining Sharif's call to engage militants in peace negotiations.
"The Prime Minister said that terrorists have no religion and targeting innocent people is against the teachings of Islam and all religions," his office said in a statement.
"He added that such cruel acts of terrorism reflect the brutality and inhumane mind set of the terrorists."
Christians make up about four percent of Pakistan's population of 180 million and tend to keep a low profile in a country where Sunni Muslim militants frequently bomb targets they see as heretical, including Christians, Sufis and Shi'ites.
"I heard two explosions. People started to run. Human remains were strewn all over the church," said one parishioner, who only gave her first name, Margrette. Her voice breaking with emotion, she said she still could not find her sister.
Police said the death toll included at least four children, two policemen and six women and was likely to rise. No one immediately claimed responsibility for the biggest attack against Christians in years.
Peshawar's deputy commissioner, Zaheerul Islam, told Geo 40 people were killed and 70 wounded in a densely populated Christian residential area near a market.
Some residents, enraged at the lack of adequate security at the historic church, took to the streets immediately after the attack, burning tyres and shouting slogans. Shops were closed in the Kohati Gate area where several other churches are located.
"Terrorists have not spared mosques, temples and churches. Please have mercy on us," one man outside the church, his face distorted by fear and anger, told Pakistan's private Geo television channel.
A bomb disposal security source said there were two explosions carried out by a pair of attackers. More than 600 parishioners were inside the church for the service.
"After the service ended, people started to come out and the suicide bomber rushed towards them," said Najeeb Bogvi, a senior police officer.
Pakistan one of the most dangerous countries in the world for minorities, and Christians, Shi'ite Muslims and Ahmadis are victims of a rising tide of deadly violence.
In 2009, 40 houses and a church were set ablaze by a mob of 1,000 Muslims in the town of Gojra in Punjab province. At least seven Christians were burnt to death.