Syrian government forces backed by Russian air strikes have seized several neighbourhoods in the ancient city of Palmyra, state TV has said.
Russian aircraft carried out 40 missions over the city in the past 24 hours, the defence ministry said.
A monitoring group said the fighting was the most intense seen so far in the Syrian army's campaign to recapture the city from so-called Islamic State (IS).
IS seized the Unesco World Heritage site and adjoining modern town in May.
It destroyed archaeological sites, drawing global outrage.
Two 2,000-year-old temples, an arch and funerary towers were left in ruins.
In a statement, Russia's defence ministry said the strikes hit 158 IS targets killing more than 100 militants.
Images released by the Syrian military showed tanks and helicopters firing at positions in Palmyra. The date of the footage could not be independently verified.
Pictures also showed repeated explosions and smoke rising from many buildings.
IS warned residents to leave the city earlier this week as the conflict intensified, the UK-based Syrian Observatory for Human Rights said.
The monitoring group said government forces had taken three neighbourhoods that are part of the modern town, AP news agency reported.
Syrian state media said earlier the army had taken full control of the al-Amiriya district on Palmyra's northern edge.
On Friday, Syria's official Sana news agency reported that troops, backed by Russian air strikes, had taken a reconstructed 13th Century castle perched on a hill to the west of the Roman-era ruins.
The castle, known as Qalaat Shirkuh or Qalaat Ibn Maan, sits on a 150m-high (500ft) hilltop overlooking the ruins and is of strategic importance, pro-government media reported.
Ancient city of Palmyra
- Unesco World Heritage site
- Site contains monumental ruins of great city, once one of the most important cultural centres of the ancient world
- Art and architecture, from the 1st and 2nd centuries, combine Greco-Roman techniques with local traditions and Persian influences
- More than 1,000 columns, a Roman aqueduct and a formidable necropolis of more than 500 tombs made up the archaeological site
- More than 150,000 tourists visited Palmyra every year before the Syrian conflict
Government forces briefly entered the town on Thursday but were pushed back.
The prospect of its liberation has been welcomed by Unesco, the UN's cultural agency, which has described the destruction of Palmyra as a war crime.
The jihadist group, which has also demolished several pre-Islamic sites in neighbouring Iraq, believes that such structures are idolatrous.
But the head of Syria's antiquities authority Mamoun Abdelkarim promised to repair as much of the damage as possible as a "message against terrorism".
In addition to its ruins, Palmyra is situated in a strategically important area on the road between the capital, Damascus, and the contested eastern city of Deir al-Zour.
Recapturing Palmyra would be a significant victory for the government and Russia, which withdrew most of its forces earlier this month after a six-month air campaign against opponents of President Bashar al-Assad.
Russia's intervention is widely seen as having turned the tide of the five-year civil war in Mr Assad's favour.