Syrian President Bashar al-Assad's forces, supported by Russian air power, are trying to retake the last large rebel-held region in Syria after nine years of war. Nearly a million Syrians have been displaced by the latest fighting.
Elsewhere in Idlib, government forces captured the town of Kafr Nabl and nearby areas some 30 km (20 miles) southwest of Nairab, according to the Syrian Observatory for Human Rights, a local source and pro-Damascus media.
U.S. Secretary of State Mike Pompeo criticized the offensive, saying the Syrian government could not achieve military victory and that Washington was working with Ankara on how to resolve the crisis. He provided no details.
Turkey has sent thousands of troops and equipment into the region to support the rebels in resisting the offensive.
"With the help of our Turkish friends, we have regained control of the strategic town of Nairab, the gateway of Saraqeb, after expelling the terrorist Russian militias," said Yusef Hamoud, spokesman for the Turkish-backed National Army.
A Turkish security official said the Turkish military had supported the rebel offensive with shelling and that bomb disposal teams and the rebels were clearing the town, located about 12 miles (20 km) southeast of rebel-held Idlib city.
Their next goal was the strategic town of Saraqeb where the M5 highway, Syria's main north-south artery linking Damascus and Aleppo, meets the road west to the Mediterranean.
Rebels said the capture of Nairab put the M5 road within range of their guns, just days after the government in Damascus declared it fully open to traffic for the first time in years.
"The capture of Nairab has restored opposition morale and the next target of the campaign is Saraqeb," said Syrian military defector general Ahmad Rahhal.
About 20 km (12 miles) south of the border, 10 civilians including seven children were killed in a Russian air strike on a shelter for displaced families in the opposition-held town of Maarat Misrin, said Yahya Jaber, a rescuer in the civil defense emergency response force.
Rebel-held Idlib city, the provincial capital, was also attacked. The Observatory said two pupils and two teachers were killed when artillery fire struck a school in the city.
U.S. SUPPORT
Since Turkey poured troops into northwest Syria to halt the Syrian government forces' campaign, 17 members of the Turkish forces have been killed.
The United States has expressed support for Turkey's stance on Idlib. "The regime's offensive only heightens the risk of conflict with our NATO ally Turkey. The answer is a permanent ceasefire and U.N.-led negotiations under U.N. Security Council Resolution 2254," Pompeo said.
The fighting has strained ties between Turkey and Russia, which back opposing sides in Syria's conflict but which had also worked to contain the violence until the latest flare-up.
Turkish President Tayyip Erdogan said there was not yet agreement on holding a March 5 summit he proposed with Russia, France and Germany on the Idlib conflict, but he may meet Russian President Vladimir Putin on that date.
Erdogan said a Russian delegation was set to come to Turkey on Wednesday to discuss the situation.
On Saturday, Erdogan said Turkey had set out a "road map" for Syria after calls with the three leaders. The Kremlin has said it was discussing the possibility of a four-way summit.
Turkey, which has already taken in about 3.7 million Syrian refugees, says it cannot handle another wave and has closed the border.
Syrian government forces are advancing closer to the camps for displaced persons near the Turkish border, where the migrants fear being caught up in the fighting.
In Geneva, the International Committee of the Red Cross (ICRC) called on the warring sides to allow safe passage for civilians to escape attacks. It reminded them that hospitals, markets and schools are protected by law.
"We are urging parties to allow civilians to move to safety, either in areas they control or across the front lines," ICRC spokeswoman Ruth Hetherington said.
Most of the displaced are cramming into pockets of territory near the Turkish border. Bitter winter weather has made their plight more desperate, with many camping by roads or in fields due to the lack of shelter.
The government offensive could mark the final chapter of a war that has killed hundreds of thousands of people, made millions of refugees, and devastated whole cities since an uprising against Assad broke out in 2011.