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BEIRUT -- Syrian rebels routinely kill captured soldiers and suspected regime informers, human rights monitors said Thursday, warning of mounting war crimes committed by those trying to topple President Bashar Assad.

Reports of rebel abuses come as the Syrian opposition appears to be gaining momentum in a 2-year-old conflict that, according to the UN, has killed more than 70,000 people.

Abuses by the Assad regime remain far more deadly, systematic and widespread, particularly attacks on civilians with imprecise battlefield weapons, including widely banned cluster bombs, the London-based group Amnesty International said.

The frequency and scale of such attacks has increased in recent months, the group said in twin reports released Thursday. The reports detail the conduct of the regime and rebel fighters. Previous reports about regime abuses received wide coverage.

Still, rebel fighters, who have generally enjoyed public sympathy in the West, must also be held accountable, said Cilina Nasser of Amnesty. "It's time for the armed opposition groups to know that what they are doing is very wrong, and that some of the abuses they committed amount to war crimes," she said.

U.S. State Department spokeswoman Victoria Nuland said the U.S. has protested human rights violations by the opposition and demanded that rebels abide by accepted standards of conduct.

"We've also made that a condition of our support," she said. The U.S. and Europe refuse to arm the rebels for fear weapons could end up in the hands of Islamic extremists.

The main Western-backed rebel group, the Free Syrian Army, denied that rebels often kill captured soldiers.

"We do not deny that it's happening, but these are individual cases, people who take revenge because their father or relatives have been killed by the regime," said Bassam al-Dada, a Turkey-based FSA official. "This happens in wars all over the place."

The FSA introduced a code of conduct for fighters, but it holds no sway over many of the armed groups, particularly the al Qaeda-linked Jabhat al-Nusra group, which dominates in key battle fronts.

The Syria conflict erupted two years ago with a largely peaceful uprising against Assad. A regime crackdown triggered an armed insurgency that by last summer had turned into a full-scale civil war.

The fighting between tens of thousands of armed men on both sides has devastated large parts of the country and forced some 4 million of Syria's 22 million people from their homes.

In a review of rebel conduct, killings of captured government fighters and suspected regime supporters emerged as the main type of abuse, Amnesty said.

Rebel groups, including some affiliated with the FSA, "are summarily killing people with a chilling sense of impunity, and the death toll continues to rise as more towns and villages come under the control of armed opposition groups," it said.

A slew of amateur videos emerged in recent months, purportedly showing such killings or their aftermath. Several show captured men being killed in a hail of gunfire.

A video said to have been taken March 9 in Raqqa, a city overrun last week by the rebels, showed three bodies lying in a city square in pools of blood. One of the dead was face down, his hands tied behind his back.

"The dogs of military security were executed in clock square," said the narrator.

Rami Abdul-Rahman of the Britain-based Syrian Observatory for Human Rights said he has received dozens of such videos and is growing increasingly concerned about the behavior of opposition fighters.

"Human rights are human rights. You cannot differentiate," said Abdul-Rahman, who relies on a network of opposition activists to provide daily updates on fighting and casualties.

Both Abdul-Rahman and Peter Bouckaert of the international group Human Rights Watch said summary executions appear common.

"Rebel abuses pale in comparison to the level of executions and other atrocities committed by the regime, but it is an issue of serious concern," Bouckaert said. "We do see quite regular cases of executions of captured soldiers and suspected regime supporters."

Investigators acknowledge that videos can be manipulated and that witness accounts and other corroborating evidence are needed.

Al-Dada, the rebel army official, alleged that videos purportedly showing rebels killing captives had been doctored, and that some in fact showed regime soldiers executing defectors or dissidents. He did not elaborate or provide examples.

Amnesty said it investigated one of the most shocking videos of recent months - what appears to show the beheading of two Syrian army officers abducted by rebels in August in the eastern town of Deir al-Zour.

Nasser said researchers contacted the families of the two men, Col. Fuad Abd al-Rahman and Col. Izz al-Din Badr.

The relatives said the kidnappers, identifying themselves as members of the armed opposition group Osoud al-Tawhid, contacted the families shortly after the abductions and demanded ransom. Negotiations ensued, but the kidnappers eventually informed the families that the two men had been killed, Amnesty said. Video of the purported killing first emerged in November.

Amnesty's Nasser said she believes the video is authentic, noting that the families identified the slain men. She said a witness in Deir al-Zour confirmed that the killing took place and provided a location, while other Syrians confirmed that those in the video spoke the local dialect.

Amnesty said there has been an increase in summary executions since the start of the conflict, but that it is impossible to estimate how many people have been killed this way.

A medical relief worker in Douma, a suburb of the capital of Damascus, told the group that in the summer of 2011, he was called to retrieve about one body every two weeks, but that by the summer of 2012, that number had risen to several a day.

Nasser quoted local rebel commanders as saying that when the regime began intensifying airstrikes last summer, summary killings increased. "In the battlefield, they felt captured soldiers would slow them down, so they would kill them," she said.

Amnesty's second report, on regime conduct, said Assad's military has intensified indiscriminate air and artillery attacks on civilian populations in recent months.

Unguided air-delivered bombs, ballistic missiles and cluster bombs are among the weapons used by the regime, "in utter disregard for the most fundamental principles of international humanitarian law," the group said.

Cluster bombs explode in the air and drop dozens of "bomblets" that often do not explode on impact. They remain explosive, increasing the threat of later injury to civilians. A majority of countries have banned their use, though not Syria.

In a recent two-week period, an Amnesty researcher visited 17 towns in rebel-held areas of northern Syria and counted more than 310 people killed in regime attacks, including 157 children and 52 women, the report said. This included more than 160 people killed in three ballistic missile attacks on the city of Aleppo, Syria's largest.

The regime either attacked civilians directly, with no obvious military targets in the area, or fired indiscriminately, the group said.