Syrian government forces have pushed into an opposition stronghold, as Turkish troops and allied Syrian fighters battle Kurdish militants in a nearby area.
Syrian troops and allied militia forces moved within nine miles of the town of Saraqeb, in Idlib, the opposition's largest stronghold in the country, a war monitoring group reported.
The Syrian Observatory for Human Rights said pro-government forces are inching closer to a key highway connecting two of Syria's largest cities, Damascus and Aleppo, which passes just east of Saraqeb.
Syria's military used its monopoly on air power to carve a path deep inside Idlib to reach the Abu Dhuhour air base, 16 miles south-east of Saraqeb, last month. It then started marching toward Saraqeb, an important military centre for rebels and al-Qaida-linked insurgents in control of Idlib.
"The bombing has been non-stop," said local media activist Abdulghani Dabaan.
The opposition has limited anti-aircraft capabilities.
Meanwhile, Turkey has mobilised some 10,000 Syrian opposition fighters to take part in its campaign against a Kurdish militant group approximately 31 miles to the north.
That campaign, codenamed Operation Olive Branch, has drawn protests from the US and France, who consider the Kurds an ally in the war on Islamic State.
Turkey considers the Kurdish People's Protection Units (YPG) to be an extension of a Kurdish insurgency inside Turkey, and views the group at its borders as a national security threat.
Ankara took umbrage at remarks by French president Emmanuel Macron, who warned against an "invasion operation".
Turkish foreign minister Mevlut Cavusoglu said this was an "insult", adding that France is in no position to "teach a lesson" to Turkey over its cross-border offensive, referring to past French military interventions in Algeria and other parts of Africa.
Mr Cavusoglu said France understood that Turkey was fighting "terrorists" and did not aim to invade Afrin.
Some 15,000 civilians have been driven by the joint Turkish-Syrian opposition campaign into the Kurdish-controlled city of Afrin, according to UN humanitarian adviser Jan Egeland.
Mr Egeland spoke in Geneva after a regular meeting of world and regional powers in a UN humanitarian "task force" for Syria.
Booby traps and mines planted by Islamic State in Raqqa have killed or wounded an average 50 people per week since US-backed fighters -- including the YPG -- expelled the jihadist group from the city in October, Mr Egeland said.
He also said that aid deliveries to "besieged areas" in Syria have fallen to their lowest level since 2015 -- before the task force was created -- with no access to them at all for the last two months.
The opposition's Higher Negotiations Committee said it is ready to back a Russian-brokered constitutional reform initiative for Syria, so long as it is led by the United Nations.
Any constitutional committee must be formed at the UN, and include representation from the HNC, which represents the Syrian opposition in UN talks with the government in Geneva, said HNC chief Nasr al-Hariri.