Shutting down of communications seen as bid to stymie rebel moves as militias attempt assault on the regime's power base

Syrian officials shut down nationwide internet access on Thursday and closed Damascus airport as rebels mounted offensives nearby and tried to advance on the capital from four directions. Phone networks were also crippled in much of the country, causing fear and confusion on both sides and fuelling claims that a new rebel push was gaining momentum.

Syria's information minister blamed "terrorists" for the outage, but the communications shutdown was seen as an attempt to stymie rebel moves as militias try to co-ordinate an assault on Damascus. It was also thought to be aimed at thwarting any plans for advances in other towns and cities.

Opposition groups have also been advancing in northern Syria, particularly near the second city, Aleppo, where the downing of two regime aircraft with surface to air missiles this week has given impetus to a rebel campaign that had become a series of attritional battles.

While officials have frequently shut down internet and mobile phone access to opposition-held areas since the uprising began in March 2011, sometimes for weeks at a time, they have never before cut web and voice communications nationwide.

Soon after noon on Thursday, all 84 of Syria's ISP address blocks were unreachable, web specialists Renesys said. Five ISP addresses did continue to function. Renesys analysts said they were used to deliver malware to anti-regime activists earlier this year, a fact that would appear to link the addresses to the government.

Landline phones began to slowly come back on line later in the day.

Throughout 20 months of insurrection, Damascus has remained a regime stronghold, with loyalist army divisions able to rout a rebel offensive in July and mount large-scale reprisals in rebel areas nearby.

Rebels have long regarded the capital as the most difficult cog in the formidable state machine they have been trying to dismantle as the early days of street protests morphed into the uncompromising civil war now ravaging the country. As night fell on Thursday, regime rocket fire reverberated from the city centre and there were sustained heavy clashes near the international airport. Two airlines, Egypt Air and the Dubai-based Emirates, said they had suspended inbound flights. Two more airlines said they were likely to follow suit. The ground radar at the airport had been turned off by early evening.

Rebel groups said fighting near the airport was the most intensive since the uprising began.

The Free Syrian Army, an umbrella group of militias that has mostly led the fight against regime forces, confirmed it had launched a big push in Damascus. Regime forces were also heavily deployed and appeared to be digging in for a fierce defence of the city.

While apparently besieged, the city does not appear to be at risk of falling soon – and spelling the end of Syrian leader Bashar al-Assad and the regime he inherited from his father, Hafez al-Assad, who laid the foundations of the ruthless police state close to 40 years ago.

Syrian officials have persistently labelled the opposition as foreign-backed terrorists. Bashar al-Assad has denied the groups now fighting him are Syrian and insists rebel groups want to replace his secular regime with an Islamic state that will widely persecute minority communities.

Rebel groups continue to rail against the regime's claims, insisting their campaign is nationalistic and aimed solely at removing a vengeful regime.

In Syria's north, the opposition is being led by the rural poor – a group almost exclusively drawn from the country's Sunni majority, which is estimated to account for at least 65% of the population.

However, since the summer, the battle in northern and eastern Syria has steadily been joined by jihadist groups, who are now playing prominent roles in most clashes with regime forces, from Aleppo to Idlib and Deir el-Zour near the eastern desert.

Chief among the groups is Jabhat al-Nusra, a mainly Syrian network of militants, many of whom have fought in Iraq. Foreign fighters are also joining the fray.

While much smaller in number than regular Free Syria Army units, which are comprised of defectors and citizens, the influence of the jihadists is being increasingly felt even in Damascus, where car bombs and suicide attacks have hit many regime targets.