One of Syria's key allies admitted for the first time on Thursday that the Assad regime was losing the ground war, as rebels told the Guardian they were occupying more territory and besieging government troops in many parts of the country.

Mikhail Bogdanov, the deputy foreign minister of Russia – which has given Bashar al-Assad unstinting diplomatic and military support – said the regime faced possible defeat to the rebels, adding with unusual candour: "One must look facts in the face."

Bogdanov said: "The tendency is that the regime and government of Syria is losing more and more control, as well as more and more territory. Unfortunately, the victory of the Syrian opposition cannot be ruled out."

Rebels said they believed the 21-month conflict had reached a decisive tipping point, with Assad's military machine no longer capable of rolling them back. "The situation is excellent. We are winning. Not just in Aleppo but the whole of Syria," said Abu Saaed, a fighter in the northern rebel-held town of El Bab.

Other key international players appear to have come to the same conclusion as Moscow. In Brussels , Nato's secretary-general, Anders Fogh Rasmussen, said: "I think the regime is approaching collapse." He said it was only a question of time before the Assad government imploded.

Others in the region, however, cautioned that the final unravelling could be prolonged and bloody. "Assad's situation is very difficult," said one senior Arab source in the region. "But he has a lot of strength. He is still getting arms and finance from Iran and his military capability is still robust."

On the ground the Syrian war remains an asymmetric one. The rebels are short of ammunition and have mainly light weapons: machine guns, Kalshnikovs, and home-made rockets. The government, by contrast, has Scud missiles – fired for the first time this week at rebels in Aleppo – as well as Sukhoi jets and attack helicopters. It also has stockpiles of chemical and biological weapons, dispersed at between 40 and 50 sites across the country – a source of growing western concern.

Nonetheless, over the past three months the rebels have acquired fresh momentum. The Free Syrian Army – as well as jihadist military outfits such as Jabhat al-Nusra, outlawed by Washington this week – have overrun a succession of Syrian army bases and military schools, and is now turning the regime's weapons on them.

"Russia sent weapons to the regime. Now we are using these same weapons to kill the regime," Saeed, from the al-Tawhid brigade, said. The FSA also believed in its cause, plus it had God on its side, he said.

The rebels have recently organised into a more cohesive fighting force. Sheikh Omar, a senior al-Tawhid commander in El Bab, said the FSA had been rebranded the National Syrian Army (NSA) at a meeting in mid-November. Its structure now mirrors that of a conventional army with numbered divisions, and on Wednesday Omar was signing new identity cards for his 1,600 troops by candlelight.

The rebels now control much of rural Syria. They are closing in on Damascus, where there has been fierce fighting in the southern suburbs, and pressing other urban regime strongholds. The government has effectively abandoned large swaths of territory, and the rebels have set up their own passport control at the Kilis border crossing with Turkey, for example – where visitors are greeted with the words: "Free Syria Repablic [sic]". Kurdish militias have established their own autonomous zones in the mountainous north-east.

In Syria's sky regime jets are visible every day. At 9am on Wednesday a Sukhoi fighter bombed El Bab, dropping three percussion bombs.

On the ground, though, it is a different story. Rebel brigades launched a co-ordinated offensive this week against a radar station in the village of Shaala, just outside El Bab. Defectors paint a picture of misery and low morale among the 150 loyalist troops trapped inside. They are forbidden to watch TV or use mobile phones, and are unaware the regime in Damascus is slowly crumbling. Officers have told them they are under attack from religious "terrorists". On Tuesday another Sukhoi jet accidentally bombed the radar station, mistaking it for a enemy target. On Thursday the rebels overran it.

In the nearby town of Azaz, opposition units have laid siege to an airbase that occupies a prominent position 20km north of Aleppo. Its precarious situation typifies the logistical difficulties now facing the beleaguered and overstretched Syrian military. The opposition controls the surrounding area, and the only way to supply the base is by helicopter – at increasing risk of being shot down.

Last week, four FSA brigades launched an operation against the base. One volunteer, Abu Doshka, got within 200 metres of the perimeter fence before he was killed by shrapnel from helicopter fire. Other rebels launched home-made shells from 4km away. "The regime can never relax," one fighter, Abu Ibrahim, said., laying out the rebels' attritional tactics.

Back in the town of Azaz, 15km away, Abu Doshka's colleagues paid tribute to his sacrifice. "He was a brave man. He was with the revolution from the start," said Abdul Faiz.

Despite reports that weapons from outside Syria are now flooding into rebel hands, many units still have only rudimentary arsenals. Instead, they rely on their own ingenuity. In an anonymous concrete house, Faiz has set up his own home-made bomb factory. His workshop is a small concrete room, next to a courtyard and a sprawling lemon tree. His materials include a large tub of aluminium powder, a sack of ammonium nitrate, imported by the government from Russia, and a bundle of fuses.

"On average we can produce two shells a day. But it's difficult work," Faiz said. He said it took him eight months to master bomb-making, adding that he experimented along the way using manuals downloaded from the internet. His home-made rockets have an impressive range of 6km and are fired from a home-made metal chute.

Near the front door lay two slightly mangled regime rockets. They had failed to explode. The large, fin-tailed shells were stamped in English and Cyrillic with the words: "The detonators are inserted."

Faiz had carefully scraped out the explosive from inside. It now sits in a small bag, and will be reused in his next bomb. The Syrian military have fired several shells at his bomb-making factory, piercing a hole in one wall, but they have failed to destroy it.

Faiz declined to say from where he had got the aluminium powder, but the name of its manufacturer was printed on the tub: the firm Eckart, based in Hartenstein, Germany. (The company's website reads: "You can get to know the many possible uses of our unique product range). Faiz said he would like to build a surface-to-air missile next. "We are thinking about it," he said.