Assad regime forces on Wednesday captured southern Idlib's Khan Sheikhoun town following two days of intense fighting with the opposition, U.K.-based war monitoring group Syrian Observatory for Human Rights said.

Syrian regime forces advanced into Khan Sheikhoun with the help of Russian airstrikes on Monday. The town had been in opposition hands since 2014. The opposition's territorial foothold in neighboring Hama province dates back to the earliest days of the eight-year-long conflict.

Earlier on Wednesday, regime forces completely surrounded the town, also encircling Morek city, where a Turkish observation post is located.

A Turkish military convoy that crossed into Syria on Monday to mobilize toward the observation point in Morek city were targeted by regime airstrikes, killing three civilians and injuring 12 others.

Shortly after the airstrike, Foreign Minister Mevlüt Çavuşoğlu warned the Assad regime against aggressive behavior.

"The regime needs to not play with fire. We will do whatever it takes to ensure the security of our soldiers and observation posts," Çavuşoğlu said.

Presidential Spokesperson Ibrahim Kalın on Wednesday repeated Ankara's warning to the regime, calling on Damascus to end violations to the Idlib agreement.

Speaking to reporters after a cabinet meeting, Kalın said it was "out of question" to close or relocate the Turkish observation post located in Morek.

Kalın also said President Recep Tayyip Erdoğan would hold phone calls with Russian President Vladimir Putin and U.S. President Donald Trump in the coming days to discuss developments in Syria.

Turkey and Russia agreed last September to turn Idlib into a de-escalation zone where acts of aggression are expressly prohibited.

The Syrian regime and its allies, however, have consistently broken the terms of the cease-fire, launching frequent attacks inside the area.

Khan Sheikhoun was targeted in a sarin gas attack in 2017 that prompted U.S. missile strikes against Syria.

An investigation conducted by the United Nations and the Organization for the Prohibition of Chemical Weapons said the Syrian regime was responsible for releasing sarin on the town on April 4, 2017. Damascus denies using such weapons.

The latest Syrian regime offensive in the northwest has forced hundreds of thousands of people to flee toward the Turkish border.

The U.S.-based Union of Medical Care and Relief Organizations (UOSSM), which supports medical facilities in the northwest, says more than 730 civilians had been killed by regime or Russian forces since late April.

The U.N. Office for the Coordination of Humanitarian Affairs has said more than 500 civilians have died in hostilities.