After many scares and several false starts, the crucial battle for Syria’s second biggest city has begun.

For more than a year the southern edges of rebel-held Aleppo have been a wasteland. Regime soldiers have been fixed in their positions several kilometres from the battered city limits, while rebels have shored up defences on their side of the ruins.

Now, three weeks into Russia’s intervention in the Syrian war, there is movement on one of the conflict’s most static fronts. And weary opposition forces don’t like what they are seeing.

“The regime advanced six kilometres [on Friday] and they took three villages,” said Zakaria Malafji, a member of the Free Syrian Army inside Aleppo. “The Russians showered us with bombs even in the civilian areas. They want to clear everything so the regime tanks and even the soldiers on foot can advance.”

Pitched against the mix of Islamists and non-ideological rebels in the rubble is the strongest force that Bashar al-Assad has been able to call on at any point during the four-and-a-half-year war. An Iranian military brigade is stationed around 20km south, along with hundreds of Hezbollah fighters, Shia militias from Iraq and the Syrian Army.

A senior US official on Friday said the Pentagon estimated the Iranian strength at 2,000 officers and soldiers – Tehran’s largest contribution to a battle and a signal that it is no longer shy to acknowledge the fact that its troops are actively defending the regime.

Straight from a grinding battle in the mountains near Damascus, Lebanon’s Hezbollah has also travelled to Aleppo en masse. “Every one of the brothers I know has gone there,” said one resident of the Beirut suburb of Dahiyeh, a Hezbollah stronghold. “This is the first time they’ve all disappeared like that. They’re even shortening their vacation times.”

Rebels inside Aleppo say they have the weapons and the stamina to keep their enemies from seizing the eastern half of the city they have controlled since July 2012. They say that large numbers of anti-tank missiles supplied by their allies – Turkey, Saudi Arabia, Qatar and the US – have reached them in recent days and warn that they have had three years to prepare their defences.

However, rebel numbers have been stretched by a three-pronged advance on the city; the attack from the south, a similar push from Idlib province towards western Aleppo, where opposition groups had been trying to besiege the regime-held area, and also from a resurgent Islamic State, which is now within striking distance of the city’s northern limits.

Malafji said that the few remaining residents of eastern Aleppo, most of whom lived in areas that had been repeatedly barrel-bombed by the Syrian air force, were now fleeing. “Civilians immediately started leaving their houses in waves and fled to the neighbouring villages,” he said. “And our people had to retreat for more support.”

Usama Abuzaid, a senior adviser to the Free Syrian Army, said: “In the last couple of days the attacks increased everywhere in the countryside, even in the areas where Isis is trying to advance.

“Aleppo is very important for everyone. For us, it is our supply line to Turkey for food and weapons. Also, it has a revolutionary value for us. It holds our main FSA headquarters, and that’s the reason the Russians are advancing.

“The regime and Isis tried to take Aleppo last year and they couldn’t, and now they are trying again with the Russians. The Russians are doing Isis a huge favour. They are giving them air cover while they are attacking us from the ground.”

Throughout the opposition lines, inside Aleppo and in Idlib to the west, where Islamists and jihadis hold sway over non-ideological units, the politics of the latest offensive and what they mean for both the war and the region are now being hotly debated. Until now Iran had tried to avoid a public show of its role in the war, fearing that it would be portrayed as a Shia invasion force, amplifying already febrile sectarian tensions.

Mohammed al-Sheikh, a senior member of an FSA-aligned unit called Division 1, said: “We know that, if the Iranians bomb us, it would be the start of the third world war. Everyone from the Arab world would come and join us, because it would be a Persian attack on Syria. This is why the regime has played it smartly, by bringing in the Russians.

Facebook Twitter Pinterest Free Syrian Army fighters carry pictures of an FSA commander and a fighter during a march to mourn their deaths in Aleppo last week. Photograph: Hosam Katan/Reuters

“For the regime, Aleppo is the economic hub. If we lose Aleppo, it doesn’t mean we lose the war, it means we lost a phase. The same is happening with the regime. Each day we win one part and then we lose another.

“Our friends understand the gravity of the situation, but their hands are tied by politics and international agreements. We are the Syrians who are losing on a daily basis because of these politics.

“If we look at who has what in Syria, you will see that Isis is only controlling the desert, and it is worthless. Yes, they have oil and water, but now we are controlling the core of Syria and that’s why the regime is crying for help.”

Syria’s allies have been determined to portray themselves as acting in support of the country’s army in battles around the country. However, the state army’s track record in numerous battles nationwide over the past year had been poor, and Iran especially had feared that the regime was teetering, as it was in late 2012, when opposition units penetrated the heart of Damascus.

Both Tehran and Moscow maintain that their now muscular interventions are aimed at countering Isis, a claim that may prove true, but not before the more potent threat to Assad – the armed opposition to him – is neutered.

In the meantime the FSA in Aleppo has given up on its earlier hopes of receiving broader support from the US. “We are confused about the Americans,” said Malafji. “We can only see the Americans active near the Kurdish areas, but not anywhere else.”

From a base in Aleppo’s southeast, not far from the city’s airport that is still used by the regime, Abdur Rahman, a long-term member of an Islamist unit, said: “We used to dream about the sound of F-16s. But for us it’s just the Syrian planes, and the deeper roar of the Russian jets. That’s our fate.”