MANBIJ - Tens of thousands of Syrian civilians have fled ferocious fighting between Russian-backed regime forces and Islamic State group militants over the past week in the country's ravaged north.

Supported by Russian air power and artillery, Syrian government forces have waged a fierce offensive against IS, seizing around 90 villages since mid-January. They took eight on Saturday alone, a military source told state news agency SANA, "expanding our control in northeast parts of Aleppo province". Their aim, according to the Syrian Observatory for Human Rights, is IS-held Khafsah, the main station pumping water into Aleppo. Residents of Syria's second city have been without mains water for 47 days after the militants cut the supply.

The fighting over the past week has sparked an exodus of "more than 30,000 civilians, most of them women and children", Observatory head Rami Abdel Rahman said Saturday.

At least 115 members of pro-regime forces and 283 militants were killed in the more than month-long fight for Palmyra, which has traded hands several times in the war, the Observatory said.

It also said 11 civilians were killed on Saturday in likely Russian raids on an IS-held village northwest of Palmyra. "The raids targeted a livestock market in the village of Oqayrabat, held by the Islamic State group in Hama province," said Abdel Rahman.

Most of the displaced went to areas around Manbij, held by the US-backed Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab fighters also fighting IS. An AFP correspondent in Manbij saw dozens of families speeding towards the relative safety of the town on motorcycles and in small buses and cars.

Many looked exhausted as they lined up at a checkpoint manned by the Manbij Military Council, the SDF unit that controls the town, to be searched and get permission to enter.

Ibrahim al-Quftan, co-chair of Manbij's civil administration, told AFP that as many as 40,000 displaced had arrived in recent days.

"The numbers of displaced people here are still rising because of the clashes between the Syrian regime and Daesh (IS)," Quftan said. "These people are suffering very difficult circumstances."

Manbij already hosts "tens of thousands of displaced people that fled previous clashes in the area and are living in difficult circumstances", according to Abdel Rahman.

"This will make it difficult (for local authorities) to welcome a new wave of displaced people, given their inability to tend to their pressing needs."

Turkey said on Saturday a MiG-23 warplane, probably belonging to the Syrian air force, had crashed on the Syrian side of the border and a search was under way for the pilot who probably bailed out and may have come down on Turkish soil.

But a local Turkish governor told Anatolian news agency a rescue team had reached the remains of the plane, which suggested it might have come down on the Turkish side of a long and wending frontier. There were no pilots in the wreckage.

“The MiG-23, believed to have been owned by the Syrian regime, crashed on the Syrian side of the border,” Prime Minister Binali Yildirim said.

Since civil war broke out in Syria in March 2011, more than half of its pre-war population has been forced to flee their homes.

The northern province of Aleppo hosts tens of thousands of displaced Syrians, many in camps near the border with Turkey.

Rebel backer Ankara sent its own troops into Syria in August to fight both IS and Kurdish units in operation "Euphrates Shield".

Turkey considers the Kurdish People's Protection Units (YPG), which makes up most of the SDF, a "terrorist" group because of its ties to outlawed Kurdish militia in southeast Turkey.

On February 23, the Turkish-backed rebels of Euphrates Shield captured the town of Al-Bab, which was IS's last bastion in Aleppo province.

They have since set their sights on Manbij, with Turkish Foreign Minister Mevlut Cavusoglu this week threatening to bomb YPG fighters unless they leave the town. The Observatory on Saturday reported escalating violence and shelling between Euphrates Shield rebels and the SDF.

More than 310,000 people have been killed since Syria's conflict erupted with protests against President Bashar al-Assad's rule, but international efforts at stemming the violence have so far failed.

Another round of UN-brokered peace talks ended Friday in Geneva, with envoy Staffan de Mistura hoping to convene another session later this month that would include the issue of counter-terrorism.

Chief regime negotiator Bashar al-Jaafari told journalists in Geneva on Saturday the new inclusion indicated the government had successfully "imposed a rational, balanced agenda" on the talks.

"The main topic we focused on more than anything else and which 80 percent of our discussions were about, was combatting terrorism," said Jaafari, who also represents Syria at the UN.

Damascus and its ally Moscow had both insisted that "terrorism" be added to the three other focuses of the negotiations: governance, elections, and a constitution.

Russia began its air war in support of Assad's forces in September 2015, and its help was instrumental in recapturing the ancient city of Palmyra from IS on Thursday.

Syrian and Russian warplanes bombed militant positions near the city on Saturday, the Observatory said, as regime forces cleared mines left by retreating IS fighters.