Syrian government forces, backed by Russian airpower, were bearing down on the Islamic State-controlled ancient city of Palmyra amid conflicting reports about the scale of their advance.
Syria's state-run television al-Ikhbariyeh reported that the regime troops had reached the heart of Palmyra.
However, the Syrian Observatory for Human Rights, a monitoring group relying on activists on the ground, denied the report.
"The regime forces are less than 1 kilometer away from Palmyra, but no one from them has entered yet the city's districts," head of the Observatory, Rami Abdel-Rahman, told dpa.
Fierce clashes are raging between the government troops and Islamic State fighters on the outskirts of the city in central Syria, according to the Britain-based watchdog.
In the air, government and Russian warplanes struck militant positions inside Palymra, which has been under Islamic State's control for nearly a year now, the Observatory said.
The Syrian army started a major offensive to retake Palmyra, a UNESCO World Heritage site, two weeks ago.
The radical group's takeover raised international fears about the fate of the city's artefacts. In August, Islamic State destroyed several famous sites in the city, including the more than 2,000-year-old Baalshamin Temple.
The al-Qaeda breakaway group is excluded from a major ceasefire that started to take effect in Syria on February 27.
The US-Russian-brokered truce has reduced fighting in the war-torn country by 85-90 per cent, US Secretary of State John Kerry said in Moscow on Thursday.
On his third visit to the Russian capital in a year, Kerry held talks with Foreign Minister Sergei Lavrov and was set to meet President Vladimir Putin.
The relationship between Moscow and Washington has frayed in recent years over the conflicts in Ukraine and Syria, but Kerry said that each of his visits have helped to mend ties.
Lavrov agreed, saying US-Russian collaboration has had positive results in Syria.
The United States and Russia have supported opposing sides in Syria's civil strife now in its sixth year.