MANILA - A Protestant church has asked the Philippine authorities on Tuesday (June 26) to release three of its missionaries they are holding for joining a human rights fact-finding mission in the war-torn southern island of Mindanao.

All three are foreigners working for the Methodist ministry in the Philippines.

Mr Tawanda Chandiwana, who is from Mutare in Zimbabwe, was arrested on May 8 for allegedly overstaying his visa. He is with the 13 million-strong United Methodist Church.

Immigration authorities, meanwhile, confiscated the passports of Mr Adam Shaw, of Brunswick, Ohio, and Ms Miracle Osman, of Blantyre, Malawi.

"We vigorously protest this treatment of our mission personnel," said Mr Thomas Kemper, general-secretary of the United Methodist Global Ministries.

"It is unconscionable that Tawanda has been held for six weeks… We are respectfully asking that these young people be allowed to leave," he added.

Human rights groups have been documenting cases of abuses since President Rodrigo Duterte placed Mindanao under martial law in May last year, after Muslim militants seized large parts of Marawi city in southern Mindanao.

The five-month siege was the Philippines' biggest security crisis in decades, killing more than 1,100 people, mostly militants.

The group Karapatan (Rights) has tallied at least 49 victims of summary executions and 22 cases of torture, purportedly at the hands of security forces.

United Nations-appointed experts said Muslim indigenous communities, in particular, have suffered widespread, human rights abuses.

"They are suffering massive abuses of their human rights, some of which are potentially irreversible," Ms Victoria Tauli-Corpuz and Ms Cecilia Jimenez-Damary, the UN Human Rights Council's special rapporteurs on the rights of indigenous peoples and internally displaced people, said in a statement last week.

Martial rule had displaced thousands of the indigenous Lumad people, and some had been killed.

The UN experts said they had information suggesting 2,500 Lumads had been displaced since October, and that Lumad farmers had been killed by military forces in December.

The government has been targeting "undesirable aliens" who have criticised not just human rights abuses in Mindanao, but also Mr Duterte's war on the narcotics trade.

Since he took office in 2016, the Philippines has been drawing international criticism for the killing of about 3,900 people in police anti-drugs operations.

Among the foreigners who have angered President Duterte is Australian nun Patricia Fox, who was spotted in April at a rally in support of human rights.

President Duterte had accused her of "disorderly conduct" and ordered that she be investigated as part of moves against "undesirable foreigners".

The government is seeking to revoke her visa for engaging in activities not allowed under the terms of her visa.