Thousands of families are fleeing the rebel-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 government 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 government forces and moderate rebels.

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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 defence ministry has denied that its aircraft carried out any strikes on the city.

"Russian aviation did not carry out any military operations, still less air strikes, in Idlib province," Igor Konashenkov, a military spokesperson, said in a statement.

Fighting in the north

At least 280,000 people have been killed and more than half of Syria's population have fled their homes since the conflict first erupted in 2011.

In recent days, fighting has especially intensified along the country's northern border where the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group is targeting a strip of rebel territory.

SOHR says thousands of civilians remain trapped there owing to the fighting.

It said rebel groups in Azaz, in Aleppo province, tried to launch a counteroffensive on Tuesday but failed when ISIL blocked the attack with a suicide bomber, killing six.

SOHR said ISIL swept towards the opposition strongholds of Marea and Azaz on Friday, forcing thousands of civilians to flee.

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 in response to shelling of a majority-Kurdish district of the provincial capital, Aleppo city, by Syrian armed groups.

SOHR said the groups continued to shell the Sheikh Maqsoud neighbourhood on Tuesday.

They have been shelling Sheikh Maqsoud for months after an advance by Kurdish forces into rebel territory west of Marea, SOHR's Abdel Rahman said.

An estimated 130 civilians have died in the shelling since February, he said.

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".

Meanwhile, US-backed Syrian fighters advanced against ISIL in the last tract of territory the group holds near the Turkish border on Wednesday, SOHR said.

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SOHR said on Wednesday the Kurdish YPG fighters made up the majority of forces involved in the attack, contradicting US officials who said the operation would be mostly comprised of Syrian Arab fighters.

Turkey is opposed to any further expansion of Syrian Kurdish influence in northern Syria, where the YPG already controls an uninterrupted 400km.

ISIL lost 18 fighters in the fighting on Monday, taking its losses since the start of the offensive on May 24 to 79, he said.

Among those killed were 24 child fighters from ISIL's Cubs of the Caliphate recruitment programme.

The US has ignored protests from its NATO ally Turkey to back the SDF, an alliance of Kurdish and Arab fighters it regards as the most effective force on the ground against ISIL in Syria.

The Pentagon has deployed more than 200 American special forces personnel to work alongside it.