Syrian army pounds Aleppo as video appears to show rebels' revenge killings

Regime forces use artillery and aircraft to attack parts of Syria's second city as Assad praises army for facing 'terrorist gangs'

Syria's government has used artillery and aircraft against targets in Aleppo as revenge killings by opposition forces underlined the brutality of the spreading conflict and the president, Bashar al-Assad, praised his army for facing what he called "criminal terrorist gangs".

Helicopters were seen operating above the northern city as Free Syrian Army (FSA) fighters claimed to have captured several of its key neighbourhoods. In Damascus, state security sources told the AFP news agency: "The army and the terrorist groups have both sent reinforcements for a decisive battle that should last several weeks."

Fighting also erupted between rebels and government forces near two Christian areas of the capital for the first time since the uprising began 17 months ago. It has since claimed an estimated 20,000 lives.

Assad's message – which marked Syrian armed forces day – was clearly intended as a morale-booster to troops overstretched by the prolonged uprising, now widely defined as a civil war, that encompasses large areas of the country. "The fate of our people and our nation, past, present and future, depends on this battle," the president said in his first statement since four of his senior security officials were assassinated in a bomb blast two weeks ago.

"My trust in you is great, and the trust of our people in you that you are the defender of its just causes," he was quoted as saying by the state news agency.

In a vivid illustration of the cruelty of the war, video footage from Aleppo showed the apparent killing of four Assad loyalists while the corpses of government militiamen in a nearby police station suggested rebels were using the same brutal tactics for which the Syrian leader's own forces have been condemned.

A clip posted on YouTube shows four militiamen being led into a crowded yard before a prolonged burst of gunfire is unleashed as people chant: "Allahu Akbar." In the video, which could not be independently confirmed, the men are identified as members of the state-run shabiha militia from the city's Berri family. Two of them were in their underwear as they were led down a flight of stairs and lined up in front of a wall. Gunmen firing with semi-automatic rifles continued shooting after their victims had fallen to the ground, their bodies piled one on top of another.

Bashir al-Haji, spokesman for the FSA's Tawhid ("Unity") Brigade, told the Guardian in a phone interview that the shootings were in retaliation for an incident on Tuesday when 15 FSA people were killed by Berri shabiha despite a truce that was supposed to be in force in that part of Aleppo.

"We were able to kill 20 of them and arrest another 50," he said. "We held a field trial for them. We have judges and lawyers who are in the opposition. They found that seven of the Berri clan were involved in killing and they decided to execute them. Others are being kept for trial after the collapse of the regime."

In a separate development, the FSA denied a report that it had acquired shoulder-launched anti-aircraft missiles that could be used to shoot down government aircraft. NBC news reported that the weapons had been delivered via Turkey. The US government has said it was not supplying lethal weapons to Syrian rebels but Saudi Arabia and Qatar have been pressing to do so and have the cash to buy them on the international black market.

"It's all a big 'if'," said Brigadier Ben Barry of London's International Institute for Strategic Studies. "If the FSA acquired a meaningful air defence capability that would reduce the effectiveness of regime attacks. But most regime attacks are by guns and artillery."

The rebels have threatened to turn Aleppo into the "grave" of the Assad government. Thousands of residents have fled and those who remain face shortages of food and fuel and the risk of injury or death. The balance of forces is still hugely in favour of the government but there are signs that the morale of the armed anti-Assad opposition is improving.

FSA leaders are careful not to reveal their strategy but Mustafa al-Sheikh, a former army general who now heads the FSA's supreme military council, told the Guardian from the Syria-Turkey border area: "The fighting is like hit-and-run. We are not aiming to get control of any city in Syria, but we want to exhaust the regime and speed up its collapse."

Another defector, from Syrian special forces, warned that more needed to be done to reach out to the country's ruling Alawite community, many of whose members fear an existential threat if the Assad regime is overthrown. "They are fearful and traumatised and they will need protection," the defector said.

The leader of Iraq's most influential Sunni tribe, Sheikh Ali Hatem Sleiman al-Duleimi, has meanwhile confirmed that members of the tribe are continuing to cross from Iraq's Anbar province into Syria. "They don't need to seek permission, economically, spiritually or financially," he said. "They are going as brothers, as humanitarians and as people lending military help." Sheikh Duleimi said security in Anbar province, which borders Syria's eastern desert, was now worse than at any time since US troops battled Islamic fighters led by al-Qaida-aligned groups there from 2004 to 2007.

Additional reporting by Lubna Naji and Mona Mahmood