Militants tied to al Qaeda took control of the last air base held by Syrian troops in a province bordering Turkey. Syria-bound Russian transport, meanwhile, received permission to fly over Greece and Iran.

Troops loyal to Syrian President Bashar al Assad's regime evacuated their positions at the Abu Zuhour air base in the Idlib province and moved to another unspecified location, according to a local TV report broadcast on Wednesday.

The UK-based Syrian Observatory for Human Rights said insurgents, including members of the Nusra Front, al Qaeda affiliate in Syria, the base during a sandstorm that covered the region and neighboring countries on Wednesday.

"After a violent attack that had been taking place since Monday, the Nusra Front and some Islamist factions control all of the Abu Duhur military airport," the Observatory said.

The fall of the base, some 50 kilometers (31 miles) south of Aleppo, is the latest in a series of setbacks for Syrian forces in northwestern Idlib province, which borders Turkey. Earlier this year, an alliance led by the hardline Sunni Islamist group Ahrar al-Sham and the Nusra Front, captured major cities and military bases in Idlib as well as the provincial capital, also named Idlib.

Russia admits to sending experts to Syria

Meanwhile, Moscow said on Wednesday that Russian military experts were on the ground in Syria. It was the first official confirmation after weeks of rumors of a Russian presence in Syria. Moscow is one of Assad's few remaining supporters.

The experts were assisting with Russian arms deliveries to Syria. The weapons are intended to help the government combat terrorism, according to Foreign Ministry spokeswoman Maria Zakharova.

A Syrian military official told Reuters news agency on Wednesday that Russian military experts had always been in Syria, adding that their presence has increased over the last year.

At least three Russian transport planes landed in Syria in recent days, US officials told the AFP news agency on Tuesday. The White House warned that a Russian military presence in Syria could spark a "confrontation" with US-led forces conducting air strikes against Islamic State targets in Syria.

The United States has also requested Greece and Bulgaria close their airspace to Russian flights to Syria. Moscow said on Wednesday that the US request amounted to "international boorishness."

While Bulgaria said it would allow Russian planes in its airspace if Bulgarian officials were permitted to inspect the planes' cargo, Russia said it received permission for Syria-bound flights to fly over Greece and Iran, Russian news agencies reported.

The Syrian civil war, which started over four years ago, has killed more than 250,000 people and driven more than 4 million from their homes, according to the United Nations.

sms/jil (AP, AFP, Reuters, dpa)