GENEVA (Reuters) - A Muslim indigenous community in the southern Philippines has suffered widespread human right abuses that could intensify with President Rodrigo Duterte’s extension of martial law there, U.N.-appointed experts said.

Duterte has called the huge island of Mindanao a “flashpoint for trouble” and atrocities by Islamist and communist rebels. He placed it under martial law in May after Islamist militants took over the city of Marawi.

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

Lawmakers this month overwhelmingly backed his plan to extend martial law there through 2018, which would be the country’s longest period of emergency rule since the 1970s era of strongman Ferdinand Marcos.

The militarization had displaced thousands of the indigenous Lumad people and some had been killed, said Victoria Tauli-Corpuz and Cecilia Jimenez-Damary, the U.N. Human Rights Council’s special rapporteurs on the rights of indigenous peoples and internally displaced people.

“They are suffering massive abuses of their human rights, some of which are potentially irreversible,” the two said in a statement late on Wednesday.

“We fear the situation could deteriorate further if the extension of martial law until the end of 2018 results in even greater militarization.”

The Philippines was obliged by international law to protect indigenous people and ensure human rights abuses were halted and prosecuted. “This includes killings and attacks allegedly carried out by members of the armed forces,” they said.

The government fears that mountainous, jungle-clad Mindanao, a region the size of South Korea, could attract foreign militants.

The U.N. experts said they had information suggesting that 2,500 Lumads had been displaced since October, and that Lumad farmers had been killed by military forces on Dec. 3 in the province of South Cotabao.

“We fear that some of these attacks are based on unfounded suspicions that Lumads are involved with militant groups or in view of their resistance to mining activities on their ancestral lands,” the pair said, without giving further details.

In Manila, opposition members of the House of Representatives filed a petition with the Supreme Court questioning the legality of extending martial law.

They asked the court to declare the extension null and void “for having been requested and granted without sufficient factual basis on the existence of an actual invasion or rebellion as required by the constitution”.

A spokesman for Duterte said the martial law extension was needed “to quell the remaining terrorists who brought destruction to Marawi and its neighboring communities”.

Its legal and factual basis had been “clearly established based on the security assessment by our ground commanders”, Harry Roque added in a statement.

Since Duterte took power in June last year, the Philippines has also drawn international criticism for the killing of about 3,900 people in police anti-drugs operations. Police deny allegations by human rights advocates that many of the killings were executions.