Syrian rebels have executed four men identified by activists as members of a pro-government tribe in the northern city of Aleppo.
The Britain-based Syrian Observatory for Human Rights says a video posted on the Internet shows the rebels executing the captured government loyalists on Tuesday. In the footage, fighters shouting "God is Great" shove the detainees against the outside of a building before opening fire with rifles. The bodies of the men can be seen lying on the ground.
The Observatory named one of the executed men as Zeno al-Barri, the head of a militia loyal to Syrian President Bashar al-Assad. Rights groups have accused rebels of carrying out multiple abuses during Syria's 17-month uprising, while reserving the strongest criticism for security forces and pro-Assad militiamen blamed for the worst atrocities of the conflict.
Related video report by Michael Bowman
Fate of Nation
Assad's whereabouts have been unknown since rebels assassinated four members of his inner circle in a Damascus bombing last month.
The Syrian president declared the "fate of the nation is at stake" in a statement to mark Armed Forces Day Wednesday.
It was the first time since succeeding his father as president 12 years ago that Assad did not deliver his message for Army Day in person or on state television.
In his message, Assad urged his countrymen to "step up the fight" against what he called "terrorist gangs," adding that Syria's enemies were using "agents inside [the country].....to weaken the government."
The terms are what Syrian government leaders use to refer to rebels and opposition figures fighting to oust Assad.
Special Report - Arab Spring: The Evolution of Revolutions
In Syria's largest city of Aleppo, battles continued to rage along the southern perimeter for a 13th day. It was not clear if either the government or the rebels had gained any ground.
Videos showed rebel fighters capturing at least two police stations from government forces on Tuesday. Witnesses say rebels executed some government prisoners, but the reports could not be independently verified.
The battle for Aleppo, the nation's commercial capital, has been one of the most critical fights in the Syrian uprising.
The U.N. observer team in Damascus reported that government forces used helicopters and fighter planes to attack Aleppo.
Spokesperson Sausan Ghosheh noted that the rebels now possess heavy weapons and that civilians are caught between both sides.
"We now have confirmation that the opposition is in possession of heavy weapons, including tanks," Ghosheh said. "The situation that concerns us the most is the situation of civilians."
Watch related video of rebel fighters in Aleppo
The United Nations said that some 200,000 civilians have fled Aleppo and surrounding areas in recent days. Those stuck inside the city are suffering from food and fuel shortages.
"There are some civilians who are trapped in the cross-fire," Ghosheh said. "There are others who are seeking refuge and shelter in schools and hospitals and public buildings in safer neighborhoods. There is a shortage of food, fuel, water and gas."
Several top rebel officers denied a report by NBC News that the opposition Free Syrian Army had acquired 20 surface-to-air missiles known as "manpads." Possession of the weapons could hinder the government's ability to bomb targets.
Riad Kahwaji, director of the Institute of Near East and Gulf Military Analysis in Dubai, says that government troops are causing widespread destruction by shelling ground targets, but that the rebels are becoming better armed and able to hold their ground.
"They're causing a lot of destruction, inflicting a lot of collateral damage, but with little apparent effect on the rebels, who are retreating and then re-attacking," Kahwaji said.
"We see the rebels making solid gains and holding onto territory and we also see an improvement in the level of equipment and weapons for the rebels up in the north," he said. "We're starting to see some heavy weapons, heavy mortars, heavy machine guns, and rocket-propelled grenades."
The Britain-based Syrian Observatory for Human Rights said at least one person was killed in fighting Wednesday between rebels and security forces in a predominantly Christian neighborhood of Damascus.
Fighting has been rare in Christian areas of the capital during the 17-month uprising against Assad's rule. The Observatory says more than 20,000 people have been killed since the uprising began.
Syria Death Map - updated August 1, 2012
Meanwhile, in a report on the government's ongoing assault in Aleppo, rights group Amnesty International says the violence is the culmination of months of a "brutal crackdown" against dissidents.
“The current onslaught on the city of Aleppo -- which puts civilians even more at grave risk -- is a predictable development which follows the disturbing pattern of abuses by state forces across the country,” said Amnesty's senior crisis response adviser, Donatella Rovera, who recently spent several weeks in Syria investigating abuses.
Amnesty says large numbers of demonstrators and bystanders have been shot dead in Aleppo by security forces and armed militias working alongside government forces. The group says families of those killed have been pressured to sign statements saying their relatives were killed by "armed terrorist gangs."
World Weighs Options
The United Nations General Assembly is scheduled to meet Thursday to discuss a Saudi resolution on Syria.
The draft expresses grave concern about the violence and the government's threat to use chemical weapons against what Syria calls foreign invaders. It also calls on Assad to step down.
There are no vetoes in the General Assembly, unlike the Security Council, which saw Russia and China veto three separate resolutions threatening Syria with sanctions for using heavy weapons against civilians.
Some information for this report was provided by AP, AFP and Reuters.