Syrian army troops backed by pro-regime militiamen entered rebel-held neighborhoods of the central city of Homs on Tuesday after laying siege to the districts for nearly two years.

The assault follows a U.N. operation earlier this year that saw some 1,400 people trapped inside the besieged neighborhoods evacuated, though around 1,300 people, mostly fighters, remained behind.

It comes a day after the regime seized the Christian town of Maalula in the Qalamun region and as media reported the country would soon move into election mode.

"The Syrian army and the (pro-regime militia) National Defense Forces have achieved key successes in the Old City of Homs," Syrian state television said.

It added that troops were advancing in several besieged neighborhoods in the area, and had "killed a number of terrorists," using the regime's term for opponents of President Bashar Assad.

Activists on the ground and the Syrian Observatory for Human Rights NGO both confirmed the operation.

"They have entered into one (besieged) area, Wadi al-Sayeh, which lies between Juret al-Shiyah and the Old City," said Abu Bilal, an activist trapped inside the blockade, who spoke to Agence France Presse via the Internet.

"This is the first time the regime has entered the besieged areas since it took Khaldiyeh" district in summer 2013, he added.

Activist Abu Fehmi, also trapped in the siege, said the army was "bombing very, very intensely".

Britain-based Observatory director Rami Abdel Rahman said regime forces began the advance a day earlier.

"The military operation began yesterday after NDF forces were deployed to strengthen the regime troops' presence," he told AFP.

Homs is Syria's third city and activists have long referred to it as the "capital of the revolution" for the large protests held there when the uprising began in March 2011.

Most of the central city is now under regime control and rebel-held pockets have been under a government siege for nearly two years, leading to dwindling food and medical supplies.

In February, the U.N. and Syria's Red Crescent evacuated some 1,500 people from the besieged areas and delivered some aid.

Evacuations from the battered districts began in February, during a U.N.-supervised humanitarian operation that saw some 1,400 people leave the blockaded rebel areas.

Activists said they fear that 400 men, including rebels and draft evaders from besieged Homs who had recently surrendered to the authorities, may be held indefinitely.

Some 100 men detained at the time of the U.N. humanitarian operation remain in the hands of the authorities, activists say.

Two weeks ago, another 300 men also left the siege, including a civilian activist who identified himself as Omar and managed to speak to AFP from a school turned detention center where he is being held.

"There was a promise that the army defectors (rebel fighters) would be released if they handed in their weapons, and they did. There was talk that we draft evaders would be released too, but till now, there is nothing," said Omar.

Today, some 1,300 people, most of them fighters, remain inside the besieged areas of Homs.

As the fighting continues, the Al-Watan daily said that the speaker of parliament would announce the date of the country's presidential elections next week.

They are expected to be held in June, before the end of Assad's seven-year term on July 17.

Assad is expected to run and win. Electoral rules require candidates to have spent the last 10 years in Syria, effectively preventing the opposition-in-exile from competing against him.

It remains unclear how Syria's government can stage elections with much of its territory under rebel control and nearly half of its population displaced inside or outside the country by the fighting.

Saudi Foreign Minister Prince Saud al-Faisal, whose government strongly backs the opposition, joined international criticism of the decision to press ahead with the vote.

"The announcement by the Syrian regime to hold elections is an escalation and undermines Arab and international efforts to peacefully resolve the crisis based on the Geneva I (peace) conference," he said in Riyadh on Tuesday.

Elsewhere, state media and the Observatory said a child was killed and at least 40 people injured in mortar fire on the capital Damascus.