Thousands of families are fleeing the opposition-held city of Idlib after the heaviest bombardment in months, coinciding with a separate increase in air strikes in Aleppo province.
The developments come as Russia faces charges that its air strikes have killed dozens of civilians across Idlib province, including in an area near a hospital.
Despite strenuous denials from Moscow, Turkey has called on the international community to rein in what it called Russia's growing military intervention in Syria.
The bombardment is part of a regime offensive to take Idlib city, the provincial capital in Syria's northwest, held by al-Nusra Front and its allies since March last year.
Al-Nusra Front is not party to a Russian- and US-brokered ceasefire that went into force on February 27 between regime forces and opposition.
Russian air strikes targeted the city overnight, killing 23 civilians, the UK-based Syrian Observatory for Human Rights (SOHR) said on Tuesday.
"The air strikes are the most intensive on Idlib since the beginning of the truce," SOHR's Rami Abdel Rahman told AFP news agency.
SOHR said dozens of civilians were also wounded in the raids on Idlib. However, the Russian defense ministry has denied that its aircraft carried out any strikes on the city.
Fighting in the north
Hundreds of thousands people have been killed and more than half of Syria's population have fled their homes since the war first erupted in 2011.
Meanwhile, thousands of civilians are trapped in Marea and Sheikh Issa after Kurdish authorities closed the main road towards the autonomous Kurdish canton of Afrin to the west, according to the UN relief agency.
"Due to the closure of the Marea-Afrin road, an estimated 7,000 civilians are effectively trapped in Marea and Sheikh Issa towns," the UN Office for the Coordination of Humanitarian Affairs (OCHA) said on Monday.
About 2,000 people had already managed to leave Marea and Sheikh Issa before the road was closed on Sunday, OCHA said.
It said about 5,000 people in total have been displaced by fighting since Friday, and the situation remains "volatile and unpredictable".
Two roads blocked
Yacoub El Hillo, the UN's humanitarian coordinator for Syria, has called on armed groups to "ensure the unhindered movement and protection of civilians trying to reach safety".
However, Kurdish authorities announced on Sunday the closure of the two roads from Afrin to Marea and Azaz.
The UN says clashes have also trapped about 165,000 civilians between Azaz and the closed Turkish border.
Pablo Marco, the regional manager of Doctors Without Borders, said on Monday that tens of thousands - many of them already displaced from other areas - were caught less than 5km from the frontline with "nowhere to go".
PHOTO CAPTION
Civilians and civil defense members look for survivors at a site damaged after Russian air strikes on the Syrian opposition-held city of Idlib, Syria, late May 30, 2016. [Reuters]
Al-Jazeera