Syrian government warplanes have pounded a rebel-held neighbourhood in the central city of Homs, killing at least three people and wounding dozens, opposition activists said.
President Bashar Assad's forces also pushed ahead in Syria's offensive on the historic central town of Palmyra that is held by Islamic State (IS).
The Britain-based Syrian Observatory for Human Rights and pro-government media said troops were about nine kilometres (six miles) west of Palmyra, which is home to some of the world's most treasured archaeological sites.
IS overran Palmyra, prized for its ancient Roman archaeological ruins, for a second time in December.
In March last year, government forces had captured the town, ending a 10-month rule of terror by the extremists.
The observatory said government forces and their allies now control hills that oversee three gas fields west of the town amid intense air strikes.
Syrian troops and their allies launched a wide offensive towards Palmyra in mid-January under the cover of Russian air strikes.
The government-controlled Syrian Central Military Media (SCMM) confirmed that troops are now a few kilometres away from the town, which is home to the Unesco heritage site for which Palmyra is famous and which has already suffered massive destruction at the hands of IS.
IS has been under pressure in Iraq and Syria over past months and the march towards Palmyra comes days after the extremists lost the northern town of al-Bab that is now held by Turkish troops and Turkey-backed opposition fighters.
Iraq forces are also on the offensive to capture the western part of the northern Iraqi city of Mosul.
SCMM said that Syrian troops captured the town of Tadef from IS on the southern edge of al-Bab, adding that military experts are now dismantling explosives and booby-traps left behind by the extremists.
Palmyra is in the central province of Homs, where violence was reported on Sunday in the provincial capital that carries the same name.
Also on Sunday, the United Nations envoy for Syria met with opposition representatives separately in Geneva, reflecting the groups' struggle to form a united front in peace talks with the Damascus government.
Staffan de Mistura met first with representatives of the opposition delegation dubbed the Cairo platform.
After the meeting, Jihad Makdissi, at the helm of the Cairo delegations, said the envoy gave them papers on "how to facilitate talks" between the various opposition groups and the government.
Mr Makdissi, a former spokesman for the Damascus government who left Syria in 2012, sought to downplay differences in the opposition, saying they were "diverse" rather than "fragmented", and could agree on technical rather than political points.
"We want to be one delegation, not a unified delegation," he told reporters.
Opposition activists said air strikes on Homs' rebel-held neighbourhood of al-Waer on Sunday came a day after the area was subjected to more than 40 air raids that killed and wounded dozens.
The air strikes appear to be in retaliation for militant attacks in the city on Saturday that killed a senior security officer and at least 31 others.
The observatory and al-Waer-based activist Bebars Talawy said the air strikes killed three people.
"Today's escalation began in the early afternoon with repeated air strikes," Talawy said via text messages from al-Waer.
The observatory said that in addition to the air strikes, al-Waer is being subjected to shelling.
The swift, high-profile attacks against the Military Intelligence and State Security offices were claimed by an al Qaida-linked insurgent coalition known as the Levant Liberation Committee.