The force, dubbed the National Defence Army, gathers together existing popular committees of pro-regime civilian fighters under a new, better-trained and armed hierarchy, the Syrian Observatory for Human Rights said.
The popular committees were originally formed to protect pro-regime neighbourhoods from rebels.
"The (regular) army is not trained to fight a guerrilla war, so the regime has resorted to creating the National Defence Army," said Observatory director Rami Abdel Rahman.
Most of the new fighters are members or supporters of the ruling Baath party, said Abdel Rahman. "They include men and women, and members of all the sects."
The new force is not connected to the pro-regime shabiha militia, which the army and security forces have deployed ever since the outbreak of an anti-regime revolt to help it suppress dissent across the country.
Members of the paramilitary force, like the popular committees before, will focus on fighting in their own neighbourhoods.
On Friday, Moscow's Russia Today reported on its website that the new National Defence Army was being set up to "defend districts against gunmen".
"The Syrian authorities are set to create ... a National Defence Army, parallel to regime forces, so that the (regular) army is freed up for combat," the website reported citing an unnamed official.
Abdel Rahman, whose Observatory relies on a network of activists and medics on the ground, said Iran was involved in building the paramilitary force.
"The paramilitary force includes an elite fighting force trained by Iran," Abdel Rahman told AFP.
"Iran has provided training to the paramilitary force's commando fighters."
Iran, Damascus' key regional ally, staunchly backs Assad and in September 2012 said its elite Quds Force, which is tasked with carrying out operations outside the Islamic republic, was giving Damascus "counsel and advice".
On the ground, an activist said the new force was already active in the central province of Homs.
"The number of regime fighters in the province has swelled in recent days, as the National Defence Army has started to come into action," anti-regime activist Hadi al-Abdullah told AFP via the Internet from the rebel-held town of Qusayr.