Nine Christian civilians have reportedly been shot dead after being forced off their truck and having their hands tied together by Islamist militants in Marawi city in the Philippines.

According to local reports, members of the militant so-called Maute group murdered the group by a check-point after they were identified as Christian.

Harrowing local television images show the Christians lying dead face-down in the grass, amid reports that villagers are afraid to move the bodies because terrorists are still in the area.

A policeman was similarly caught at a checkpoint set up by the militants and beheaded on Wednesday, President Rodrigo Duterte said.

News of the killings come as chaos descended on the Muslim-majority Marawi city, where the Filipino army has sent in about 100 soldiers, including US military.

Thousands of citizens have fled after the Maute group torched several buildings and occupied various parts of Marawi. Philippines President Rodrigo Duterte has imposed martial law on Mindanao Island, which includes Marawi, in an attempt to curtail the chaos. He promised the military rule would be 'harsh'.

'I warned everybody not to force my hand into it,' Duterte said, according to The National. 'I have to do it to preserve the republic.'

Attention has also been focused on the kidnapping of a local priest and his parishioners who were taken hostage by the Maute group on Tuesday.

The president of Catholic Bishops Conference of the Philippines appealed for prayers for the hostages. He said: 'Fr Chito Suganob and others were in the Cathedral of St Mary's when members of the Maute fighting group forced their way into the cathedral, taking with them Fr Chito and others as hostages. They have threatened to kill the hostages if the government forces unleashed against them are not recalled.'

After the kidnapping the militants reportedly set fire to the cathedral and the bishop's residence. Black flags markings allegiance to ISIS have also reportedly been erected across the city.

Cotabato Archbishop Orlando Cardinal Quevedo told ABS-CBN News: 'I pray for the safety of all the hostages. I appeal to the consciences of the hostage takers not to harm the innocent as the Islamic faith teaches. I appeal to religious leaders of Islam to influence the hostage takers to release the hostages unharmed.

'For God's will is the safety of innocent people. May the loving God protect the people of Marawi.'

Bishop Edwin De la Peña, Prelature of Marawi Diocese, told Radio DZMM: 'At the time of his capture, Fr Chito was in the performance of his ministry as a priest. He was not a combatant. He was not bearing arms. He was a threat to none. His capture and that of his companions violates every norm of civilised conflict.'

De la Peña told Agenzia Fides that people in the city were 'terrified'. At least 21 have been killed since fighting began on Tuesday, after a failed government raid on a suspected terrorist hideout.

That raid was meant to capture Isnilon Hapilon, a leader of the terrorist group Abu Sayyaf. A military spokesman said he was still at large in the city.