Fighting in northwestern Syria over the last three days has killed 27 civilians, including eight children, a United Nations official said on Friday, in some of the worst violence in years between Syrian government forces and rebels.

Rebels led by the Islamist militant group Hayat Tahrir al-Sham launched an incursion on Wednesday into a dozen towns and villages in the northwestern province of Aleppo, which is controlled by Syrian President Bashar al-Assad's government.

The Syrian military said it continued to confront the attack and that it had inflicted heavy losses on the insurgents in the countryside of Aleppo and Idlib.

Insurgents breached the city of Aleppo, Syria's second largest, after blowing up two car bombs on Friday, the Syrian Observatory for Human Rights, a war monitor, reported.

Turkey state-run Anadolu Agency also reported that the opposition insurgents entered Aleppo city centre Friday. It said the insurgents "broke through the defence lines of the regime forces along the Hamdaniyya, New Aleppo, and Zahra axis on the outskirts of the city." It added the insurgents now control approximately 70 locations in Aleppo and Idlib provinces.

Assad's whereabouts unclear

David Carden, UN deputy regional Humanitarian co-ordinator for the Syria crisis, said: "We're deeply alarmed by the situation unfolding in northwest Syria."

"Relentless attacks over the past three days have claimed the lives of at least 27 civilians, including children as young as eight years old," he told Reuters.

"Civilians and civilian infrastructure are not targets and must be protected under international humanitarian law."

Syrian state news agency SANA said four civilians including two students were killed on Friday in Aleppo by insurgent shelling of university student dormitories.

It was not clear if they were among the 27 dead reported by the UN official.

The attack was the biggest since March 2020, when Russia and Turkey, which supports some of the rebels in the northwest, agreed to a deal that de-escalated the conflict.

Russian and Syrian warplanes bombed the area near the border with Turkey on Thursday to try to push back an insurgent offensive that has captured territory for the first time in years, Syrian army and rebel sources said.

Kremlin spokesperson Dmitry Peskov said Russia, which has forces in Syria backing Assad, regarded the rebel attack as a violation of Syria's sovereignty and wanted the authorities to act fast to regain control.

"As for the situation around Aleppo, it is an attack on Syrian sovereignty and we are in favour of the Syrian authorities bringing order to the area and restoring constitutional order as soon as possible," said Peskov.

Asked about unconfirmed Russian Telegram reports that Assad had flown into Moscow for talks with Russian President Vladimir Putin, Peskov said he had "nothing to say" on the matter.

In a statement, Turkish Foreign Ministry spokesperson Oncu Keceli said that avoiding greater instability in the region was the government's priority, adding that Ankara had warned that recent attacks on Idlib, a rebel-held region in the northwest, undermined the spirit and implementation of de-escalation agreements.