Manila, Philippines - Diego swam across the chilly waters of Lake Lanao in pitch darkness to make it to the opposite shore before first light.

It was August 4, and the young father was fleeing the main battle zone in Marawi, a city on the southern island of Mindanao in the Philippines, where, for three months, he was held hostage by Maute fighters linked to ISIL.

A bloody battle between the fighters and government forces was raging in Marawi since May, pushing thousands of residents to flee.

"I was crying all the time. There was always a gun pointed at me," Diego, who used a pseudonym for fear of retribution, told Al Jazeera.

"I lost count how many times they shot at me and beat me."

Diego thought his ordeal was over when, after swimming for two straight hours, military officers found him on the other side of the city.

Instead, however, he was brought to a university campus where he said he was blindfolded and shackled.

For the next week, Diego said he endured physical, mental and emotional torture at the hands of security officers, whom he said were trying to make sure he was a hostage and not one of the Maute fighters himself.

"They taunted me, saying 'Let's just kill him.' I could hear them electrocuting the guy next to me. They beat me up, thinking I would admit I was a fighter," he said.

War crimes

Diego's story has now been corroborated by a new report by human rights group Amnesty International, issued nearly four weeks after Philippine authorities declared total victory over the Maute fighters in Marawi.

The report, released on Friday and titled "The Battle of Marawi: Death and Destruction in the Philippines", describes widespread abuses committed by both Maute fighters and government forces - some of which amount to war crimes.

Based on interviews with 48 survivors and witnesses, as well as local leaders, journalists and activists, the report found that pro-ISIL fighters engaged in unlawful killings and hostage-taking, while government forces mistreated people in their custody.

Government shelling has caused widespread destruction in the city, civilians have been trapped in the crossfire and all parties to the conflict have engaged in looting, Amnesty found.

The group said it had "documented a variety of serious violations of international humanitarian law by all parties to the conflict".

"Some of these violations amount to war crimes," it added.

'Extrajudicial killings'

On May 23, Maute fighters launched an attack on Marawi, the largest Muslim city in the predominantly Catholic nation, targeting Christians or anyone who could not recite the Shahada, the profession of the Muslim faith.

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They took their victims hostage or killed them on the spot, shooting them with pistols or rifles, or slitting their throats, Amnesty found.

The report documented at least 25 of these "extrajudicial killings".

For the hostages, it was a daily struggle to survive torture and abuse at the hands of their Maute captors, Amnesty stated.

They foraged for food, retrieved and buried dead bodies, dug foxholes for the attackers, and made improvised bombs.

The men were forced to fight government forces while the women were used as "sex slaves," the report found.

Many hostages attempted to escape, but those who were caught were either shot or beheaded by the fighters, Amnesty said.

'At least you are alive'

Hostages who successfully fled their captors, like Diego, often had to deal with government forces, who were suspicious of their potential links to the Maute fighters, the report found.

Several survivors recounted being shot at, beaten up and tortured by members of the Philippine Marines in an effort to get them to confess that they were members of ISIL, Amnesty reported.

One man said he was doused in hot liquid.

After two weeks in torturous detention, a survivor told Amnesty that a police investigator apologised for what he went through, telling him: "At least you are alive. Don't complain".

Witnesses said at least 10 hostages were also killed in a government air raid.

The government said more than 1,000 people - 920 Maute fighters, 165 soldiers and police, and 47 civilians - were killed in the fighting. It also said it had rescued 1,780 hostages.

According to Amnesty, however, the government's tally of civilian casualties may be incomplete.

Military investigating

The report questioned whether civilian deaths and the "large-scale destruction" of the city in government air and ground attacks were "militarily necessary and proportional to the threat posed" by the fighters.

The fact that bombings were carried out after the military said only a few Maute fighters were still alive casts doubt on whether the operations met the requirements under international humanitarian law, the report stated.

Police and military officers were among those who looted and ransacked homes and shops left behind by fleeing residents, according to witnesses quoted in the report.

Government forces are held to a higher standard than the armed group they were fighting, said Kathline Tolosa, of the Security Reform Initiative, a Manila-based watchdog.

"The Armed Forces of the Philippines (AFP) is the protector of the people and the state … That is why we hold them to account," Tolosa said.

Restituto Padilla, a spokesperson for the AFP, acknowledged the "allegations" levied in Amnesty's report, but said the armed forces were still investigating.

"Our actions in the Marawi conflict were guided by the rules of conflict, which provides for the necessity and proportionality in the use of force," Padilla said.

"We also understand the manner in which they question our use of bombings."

Reparations

Amnesty called on the Philippine government to fully investigate the reported incidents and make reparations to all the victims who suffered from human rights violations.

It urged the government to consider lifting martial law on the island, which Philippines President Rodrigo Duterte imposed on the Mindanao region in May after the fighting in Marawi first broke out. Martial law is set to expire at the end of the year.

The human rights group also called on local authorities to rehabilitate the war-torn city as quickly as possible and provide support to displaced residents.

Diego, meanwhile, said what he experienced in Marawi has left him traumatised.

"I will never go back to Marawi unless I carry a gun. I want to become a soldier. I want vengeance," he said.