Turkish and Syrian troops traded deadly fire in Syria's northwest on Monday, further raising tension between Ankara and regime backer Moscow over the war-torn Idlib rebel enclave.

An air strike also killed at least nine civilians in the same area, where the latest government offensive has caused one of the nine-year-old Syrian conflict's worst waves of displacement. The tit-for-tat shelling between Damascus and Ankara was the deadliest since Turkey deployed troops in Syria in 2016 and escalated tensions between the conflict's two top foreign brokers.

Turkish President Recep Tayyip Erdogan had offered rare criticism of Russia last week, accusing it of "not honouring" agreements to prevent a regime offensive on the northwestern region of Idlib.

The overnight clash began with regime shelling on Turkish positions in Idlib, hours after a Turkish military convoy of at least 240 vehicles entered northwest Syria, according to the Syrian Observatory for Human Rights, a Britain-based war monitor.

The attack killed four Turkish soldiers and wounded nine others despite previous coordination on where Ankara's forces would be in the region, Turkey's defence ministry said. The Russian defence ministry said Ankara had failed to give prior warning of its troop movements at the time of the incident.

Retaliatory rocket attacks by Ankara on regime positions later killed at least 13 Syrian government troops and wounded 20 others in Idlib and the neighbouring provinces of Hama and Latakia, the Observatory said.

Most Syrian troops were killed south of Saraqeb, a flashpoint Idlib town that Damascus has been trying to encircle in recent days, said the Observatory. State news agency SANA said the Syrian army had not suffered any casualties.

'Ongoing operation'

Speaking to reporters at an Istanbul airport before leaving for Ukraine, Erdogan called it an "ongoing operation," and said that 30 to 35 Syrian forces were "neutralised" in the counterattack that targeted 40 locations.

"When we have martyrs on our side, it is not possible for us to remain silent," said Erdogan, warning Russia not to stand in the way of its response.

A senior Turkish official said that the regime attack on Turkish troops and the government offensive in general were conducted "with Russian protection".

"Russia should remove this protection from the regime elements which attack Turkish forces in this region," ruling party spokesman Omer Celik told CNN Turk. "Our target is not Russia," he said. "The regime in this region after this attack is a target for us."

Heightened attacks by the regime and Moscow in northwest Syria have displaced more than 388,000 people since December, according to the United Nations, forcing many of them towards Turkey's border.

Ankara -- which already hosts more than three million Syrian refugees on its soil -- fears the latest fighting will lead to another mass influx. UN Secretary-General Antonio Guterres on Saturday called for an end to fighting but air strikes, many of them carried out by Russian aircraft, have continued.

On Monday, at least nine civilians were killed in an air strike on northwest Syria where similar raids a day earlier had killed 14 civilians, according to the Observatory. Monday's casualties were all displaced people who died when a missile hit their car, the Observatory said, without specifying the origin of the strike. They died leaving a flashpoint region in Aleppo province, located in the Idlib region's eastern flank.

Regime push

The regime now controls more than two thirds of Syria, up from a barely a fifth before Russia's military might came to the rescue in 2015.

A jihadist group led by members of Syria's former Al-Qaeda affiliate still controls swathes of territory in and around Idlib province home to some three million people and pro-Ankara rebel groups are also present.

In recent months, pro-Damascus forces have pressed northwards along the M5 highway that connects the capital to second city Aleppo in the north, crossing Idlib. Government forces last week retook the key town of Maaret al-Numan along the highway and are now just a few kilometres from the abandoned town of Saraqeb.

But Turkish forces who deployed west of Saraqeb on Sunday are now blocking regime attempts to encircle the town and access surrounding highways, the Observatory said. A Turkish-Russian deal in 2018 saw Turkish troops deploy at observation posts around Idlib, but the agreement has failed to stem repeated regime military offensives.

Half of the Idlib region's residents have been displaced throughout the war, with many living in precarious shelters in the countryside along the Turkish border. On Sunday, hundreds of Syrian men, women and children marched towards the border in a symbolic protest, demanding to be allowed through.