Turkey's President Recep Tayyip Erdogan has threatened to launch an operation in Syria's Idlib region by the end of the month if Damascus fails to withdraw behind Turkish military positions.

"An operation in Idlib is imminent," Erdogan told his party's legislators in Parliament on Wednesday. "We are counting down, we are making our final warnings".

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Russia - the Syrian government's main ally in the nearly nine-year war - responded by saying any Turkish offensive against Syrian forces in Idlib would be the "worst scenario".

"If we are talking about an operation against the legitimate authorities of the Syrian Republic and armed forces of the Syrian republic this would, of course, be the worst scenario," said Kremlin Press Secretary Dmitry Peskov, adding Russia and Turkey were staying in contact to prevent tensions escalating further.

Ankara, which backs several rebel groups in northwest Syria, has been outraged since recent Syrian government attacks in Idlib province killed 13 Turkish military personnel in two weeks.

'No matter the cost'

Erdogan said talks with Russia over the last remaining rebel stronghold in Syria had failed to end in agreement and warned a military operation was just a "matter of time".

He said Turkey was determined to make Idlib a secure zone "no matter the cost", even as talks continue with Russia, which backs Syrian President Bashar al-Assad's forces.

"We will not leave Idlib to the [Syrian] regime, which does not understand our country's determination, and to those encouraging it," said Erdogan.

Erdogan's comments came as forces loyal to al-Assad pressed an offensive on the country's last major rebel enclave.

About 900,000 people have been forced from their homes and shelters in less than three months, including some 500,000 children, since Syrian government troops renewed the offensive on the region.

Erdogan's announcement comes a day after dire warnings by United Nations officials who warned of a humanitarian disaster in northwest Syria.

Nearly 300 civilians have been killed in attacks this year in the region, with 93 percent of the deaths caused by Syrian and Russian forces, according to the UN's human rights chief.

President al-Assad pledged this week to continue the offensive, saying the war was not yet over but a "complete victory" was in sight.

In September 2018, Turkey, Russia and Iran - the main international players in the conflict - agreed to turn Idlib province into a de-escalation zone where acts of aggression were prohibited while all sides were allowed to set up military observation posts.

However, all sides have blamed each other for violating the terms of the agreement, as multiple ceasefires last summer failed to hold.

Russian Minister of Foreign Affairs Sergey Lavrov told a news conference on Wednesday that Syrian government forces were upholding previous agreements on the region but also reacting to provocations. Lavrov said rebel attacks on Syrian and Russian forces in Idlib were continuing.

He also said the Russian and Turkish delegations failed to reach an agreement at talks in Moscow earlier in the week aimed at easing the tensions in Idlib.

Calls for restraint

Syrian aid workers issued an urgent call on Wednesday for a ceasefire and international help for nearly a million people fleeing the onslaught.

At a news conference in Istanbul, the Syrian NGO Alliance said existing camps are overcrowded and civilians are forced to sleep in the open amid a bitter winter cold.

"We are facing one of the worst protection crises and are dealing with a mass movement of IDPs [internally displaced persons] who have nowhere to go," the Syrian NGO Alliance said in a statement.

They are "escaping in search of safety only to die from extreme weather conditions and lack of available resources," it added.

More than 500,000 children have been displaced by the violence, UNICEF said in a statement. Since the beginning of the year, 77 children have been killed or injured.

"The situation in the northwest is untenable, even by Syria's grim standards," said Henrietta Fore, UNICEF's executive director.

"Children and families are caught between the violence, the biting cold, the lack of food and the desperate living conditions ... It is time for the guns to go silent and for the violence to stop once and for all."

Symbolic message

A Syrian commercial flight landed at Aleppo airport on Wednesday from Damascus, marking the resumption of internal flights between Syria's two largest cities for the first time since 2012.

The flight carrying Syrian officials and journalists was a symbolic message from al-Assad's government, days after its forces consolidated control over the northwestern province of Aleppo and seized the last segments of the strategic M5 highway linking Aleppo to Damascus.

The motorway between Syria's two biggest cities was being repaired and was scheduled to reopen in coming days, for the first time in eight years.

The Syrian Air flight landed at Aleppo airport after a 40-minute flight from Damascus and was received on the ground by a military band at the tarmac.