Saraqeb, near the Turkish border, has been at the centre of intense fighting after the regime took over Maarat al Numan - both cities dot a strategic highway linking Damascus to Aleppo.
Turkey has sent reinforcements to Idlib as Syrian regime troops entered Saraqeb, a strategic town in the province.
Turkish Armed Forces sent commando units to military observation points in a nearly 150-vehicle-strong convoy dispatched to Idlib province.
The town of Saraqeb, near the Turkish border, has been at the centre of intense fighting for days.
Regime news agency SANA and al Ikhbariya TV said Syrian regime's army units entered the town before nightfall and were combing it for remaining fighters and explosives.
On Wednesday, Turkish troops stationed north of Saraqeb shelled Syrian regime soldiers north and west of the town and the opposition counterattack successfully pushed Syrian troops back to the southern edge of town, the Syrian Observatory for Human Rights said.
Turkish President Erdogan has threatened to use force to compel regime leader Bashar al Assad's forces to retreat by the end of the month.
The Turkish Defence ministry on Friday said all attacks will be retaliated.
The push by regime forces into towns and villages in the northwestern Idlib province has caused the displacement of more than a half a million people in just over two months, compounding a humanitarian disaster in the region packed with internally displaced people.
It also risked a military confrontation between Turkish and Syrian regime troops.
The town sits at the intersection of two major highways, one linking the capital, Damascus, to the north and another connecting the country’s west and east.
Turkey has set up military posts to protect Saraqeb against the advance.
The Britain-based Observatory said army units entered Saraqeb but had not yet taken control.
Videos showed opposition and rebel fighters lobbing shells at regime troops from around the town of Binnish, northwest of Saraqeb. Fighters stacked artillery shells and burned tyres.
“Today, we are burning tyres for the war that’s hitting the town of Binnish," said one unidentified fighter, pledging to defeat anyone who tried to advance on the town. "Let them come here and we will stomp on their head one after another," he added.
The regime advance came after an attack by the regime on Monday killed seven Turkish soldiers and a Turkish civilian member of the military.
In Moscow, Russia's Foreign Ministry announced that Russian soldiers had been killed in a northern Syrian province alongside Turkish servicemen, without saying when the incident occurred or how many were killed.
The ministry blamed “terrorists” for the deaths.
The regime troops have been advancing since December into the country's last opposition and militant stronghold, which spans Idlib province and parts of nearby Aleppo. Turkish troops are deployed in some of those areas to monitor an earlier ceasefire that has since collapsed.
Syrian jets hit air base
The observatory said Syrian warplanes on Thursday attacked a military air base in the village of Taftanaz, where Turkish troops deployed recently.
The observatory said Syrian opposition later launched an attack under the cover of intense Turkish artillery shelling on the village of Nairab, which Syrian troops captured earlier in the week.
The rebels carried out at least one suicide attack with an armoured personnel carrier rigged with explosives, according to citizen journalist Taher al Omar, who is embedded with the opposition on the edge of the village.
Turkish leaders have repeatedly called on Russia to "rein in" Syrian regime forces.
Kremlin spokesman Dmitry Peskov responded Thursday that Russia was concerned about the concentration of insurgent groups in Idlib and their "continuing activity".
Turkish President Recep Tayyip Erdogan has brought up the possibility of direct contact with Russian President Vladimir Putin to discuss the situation in Syria.
Meanwhile, Turkish Foreign Minister Mevlut Cavusoglu said a delegation from Russia would arrive in Turkey on Saturday for talks on the situation following the attack on Turkish soldiers.
He said a follow-up meeting between Erdogan and Putin could also take place "if there is a need".
'Grave consequences for civilians'
UN special envoy for Syria, Geir Pedersen, called for an urgent and sustained ceasefire, restrictions on support for terrorist groups, “serious exploration of a strengthened international presence with the consent of Syrian authorities,” and a serious effort to tackle the issue of foreign fighters.
“What we are now seeing creates the very real prospect of a bloody and protracted last stand on the Turkish border, with grave consequences for civilians – and the risk of dispersal of foreign terrorist fighters and ongoing insurgency afterward,” he warned at an emergency meeting of the UN Security Council called by Britain, France and the United States.
At least 53 health facilities have suspended work in the area, according to the World Health Organization.
An urban frontline
UN humanitarian chief Mark Lowcock said the “humanitarian catastrophe” has substantially escalated as shelling and air strikes on dozens of communities force civilians "to move north and west into the ever-smaller enclave controlled by non-government groups".
That area is now "dramatically overcrowded" and "needs are growing exponentially", with hundreds of thousands of civilians at risk, in addition to those who have fled since December, he said.
"Time is short. The front-line has now moved to within a few kilometres of Idlib city, the largest urban centre in the northwest," Lowcock said.
Idlib province is home to some 3 million people, many of them displaced from other parts of Syria in earlier bouts of violence.
"Bombings and other attacks on civilians in north-west Syria must stop," EU foreign policy chief Josep Borrell and the bloc's Crisis Management Commissioner Janez Lenarcic said in a statement on Thursday.
It said the EU, a major aid donor, would continue to provide help but that "unimpeded, safe and secure access" was required.