Syrian government forces advanced Saturday into Palmyra with heavy support from Russian air strikes, taking control of several districts in a major assault against Islamic State group fighters, Syrian state media and a monitoring group said.

Television footage showed waves of explosions inside Palmyra and smoke rising from buildings, as tanks and armoured vehicles fired from the outskirts.

The Syrian Observatory for Human Rights said it was the biggest assault in a three-week campaign by the Syrian army and allied militia fighters to recapture the desert city and open up the road to Islamic State group strongholds further east.

Observatory director Rami Abdulrahman said Syrian soldiers and allied militias had taken control of one-third of Palmyra, mainly in the west and north, including part of the ancient city and its Roman-era ruins. Soldiers were also fighting on a southern front, he said.

Syrian media and Arab television channels broadcasting from the slopes of Palmyra's medieval citadel, one of the last areas of high ground seized by the army Friday, said troops had advanced inside Palmyra and had taken several neighbourhoods.

The recapture of Palmyra, which the Islamist group seized in May 2015, would mark the biggest reversal for Islamic State group in Syria since Russia's intervention turned the tide of the five-year conflict in President Bashar al-Assad's favour.

The group and al Qaeda's Syrian branch the Nusra Front are excluded from a month-long cessation of hostilities that has brought a lull in fighting between the government and rebels battling Assad in western Syria.

The limited truce has allowed some aid deliveries to get to previously inaccessible areas under siege. The International Committee for the Red Cross said on Saturday it brought food and hygiene items to three areas of south Damascus the day before.

Peace talks have also resumed in Geneva, but progress has been slow, with the government and its opponents disagreeing fundamentally on the terms of a political transition, including whether Assad must leave power.

Russia, which sponsored the talks along with the United States, has withdrawn some forces from Syria but has strongly supported the Palmyra offencive, saying this week that a Russian special forces officer was killed in combat near the city.

Russian news agencies quoted the defence ministry in Moscow as saying Russian jets had made 40 sorties around Palmyra in the last 24 hours, hitting 158 Islamic State group targets and killing more than 100 militants.