BEIRUT -- Scores of muddied and waterlogged gunshot victims, most of them men in their 20s and 30s, were found dead Tuesday in a suburb of Syria's contested northern city of Aleppo. Insurgents and the government accused each other of carrying out the killings in what appeared to be the latest civil war atrocity.
Video posted by opponents of President Bashar Assad seemed to show that many had been shot in the back of the head while their hands were bound.
The Syrian Observatory for Human Rights, an activist organization based in Britain with a network of contacts in Syria, said at least 50 bodies had been located, some scattered along the banks of a small river in the Bustan al-Qasr neighborhood, which is mostly under rebel control. Later reports put the tally much higher.
Al-Jazeera quoted a commander from the insurgent Free Syrian Army, identified as Capt. Abu Sada, as saying there could be more than 100 bodies, with many still submerged in the river's murky water, and that all the victims had been "executed by the regime."
Syria's state news agency, SANA, later posted a report on its website that blamed the insurgent Islamist fighters of the Al Nusra Front, and said the killings added to "a series of brutal massacres perpetrated by the terrorist groups against unarmed civilians."
The videos emerged as the United Nations reported a sharp increase in the number of refugees known officially to have fled Syria, increasing the total in neighboring countries to more than 700,000 from 500,000 in December.
In Washington, President Barack Obama announced that the United States would donate a further $155 million to aid Syrian refugees, bringing the U.S. total pledged as humanitarian aid to $365 million. But that is a fraction of the $1.5 billion that the United Nations says is needed over the next six months as it issues urgent appeals for more international funding.
At the same time, rebel fighters seeking Mr. Assad's overthrow appeared to have made advances in Syria's east, raiding a security office in the eastern city of Deir el-Zour, where government forces have seemed to reduce their presence to concentrate on the center, giving rebels more freedom to maneuver and, in some cases, to siphon fuel from gas and oil fields there.
Activists said the insurgents in Deir el-Zour included Islamist fighters from the Al Nusra Front, which the United States regards as a terrorist organization. The rebels freed 11 detainees and captured a tank and three armed personnel carriers, according to the Local Coordination Committees, an anti-Assad activist network in Syria.
The Syrian conflict began in March 2011 as a peaceful protest but has since spiraled into civil war.
In Geneva, the U.N. High Commissioner for Refugees said Tuesday that there had been an "unrelenting flow of refugees" across Syria's borders, principally into Jordan, Lebanon, Iraq and Turkey. The highest numbers were in Turkey, Jordan and Lebanon, but smaller numbers had been registered in Egypt and North Africa, refugee agency spokeswoman Sybella Wilkes said.
The total now exceeds 700,000, made up of around 580,000 registered and the rest waiting to be registered as refugees. The spurt in refugees meant that 200,000 have fled in less than two months since early December. "We are trying to clear a backlog of people because the numbers have gone up so dramatically," in Jordan and Lebanon particularly, Ms. Wilkes said.
Violence also flared Tuesday in central Damascus, where a car bomb exploded and severely injured a member of Parliament, Abed al-Razaq Katan, the Syrian Observatory for Human Rights reported. State news media did not immediately report the explosion.
At the U.N., Lakhdar Brahimi, the joint U.N. and Arab League envoy trying to mediate a settlement, gave the Security Council a pessimistic assessment. Echoing the themes of his last briefing in November, Mr. Brahimi said it was time for the council to act in such a way that removes any ambiguity about its wish for a political, not a military, solution.
In Geneva last June, the major powers agreed that Syria should be governed by a transitional government, but they never defined its powers. To the United States, it meant that Mr. Assad should resign. But Russia, Syria's main international ally, maintained that the Syrians must decide.
The wording in the Geneva accord was "creative, clever ambiguity," Mr. Brahimi told reporters, saying it was time for the Security Council "to say what these full executive powers mean." With the council deadlocked, Syria grows as a threat to the whole region, which should spur the council to action, he said.
Russia was hosting a dinner Tuesday night for Mr. Brahimi with the ambassadors from the five permanent Security Council members, but there seemed to be little hope for a breakthrough. Russia and China have together vetoed three Security Council resolutions focused on Syria since the conflict erupted.
"The same issues that have stymied the council to date remain unresolved, so there is no obvious way forward," said Susan E. Rice, the U.S. ambassador to the United Nations.