BEIRUT - At least 35 Syrian soldiers were killed overnight when an army cargo plane crashed in bad weather in the northwest of the country, a monitor said on Sunday.

The Syrian Observatory for Human Rights said the plane crashed after hitting electricity lines in heavy fog in Idlib province, a claim echoed by Syrian state media. But Syria's Al-Qaeda affiliate Al-Nusra Front claimed on Twitter that it had shot down the aircraft. Syrian state media reported the crash without giving a death toll.

"A medium-sized cargo plane crashed last night while landing at the Abu al-Duhur military airport because of bad weather conditions and heavy fog, killing the crew on board," state news agency SANA said. It gave no further details on the incident. The Observatory said the plane was carrying troops as well as military equipment and ammunition.

It said regime forces were involved in heavy clashes with Al-Nusra fighters in a bid to reach and recover the bodies of those killed in the crash.

On one of its Twitter accounts, Al-Nusra claimed it had shot down the plane and posted photos it said showed some of those killed in the crash, as well as the remains of the aircraft.

Militant groups have regularly shot down regime planes and helicopters in the country's civil war. In December, Islamic State group militants captured a Jordanian pilot whose plane crashed while participating in the US-led coalition against the jihadist group. IS claimed to have shot down the plane, though Jordanian and US authorities say the crash was an accident.

Meanwhile, Israel on Sunday laid charges against seven Arab Israelis over allegations they set up a militant cell linked to the Islamic State group, the Shin Bet internal security agency said.

The seven, who all come from Israel's northern Galilee region, were all arrested in November and December and indicted at Haifa District Court on Sunday. They are aged between 22 and 40.  Shin Bet said they were indicted on charges of "membership of, and activity within an illegal organisation, support for a terror organisation and attempts to make contact with an enemy agent".

In September, Israel outlawed both IS and the Al-Qaeda affiliated Abdullah Azzam Brigades as "illegal organisations", allowing for legal measures to be taken against them or anyone supporting or financing them.

In a statement, Shin Bet identified the ringleader as 40-year-old Adnan Alaaeddin and said the group met frequently with a radical Salafist cleric.

"Adnan presented himself to the other members as an senior commander within the Islamic State group in Palestine and he was a dominant figure among the activists. During their meetings, Adnan would urge them to carry out military operations and attacks on Jews," it said.

Another suspect, Karim Abu Salah, 22, was arrested at Ben Gurion airport near Tel Aviv as he was heading to Syria to join the ranks of the jihadists, it said.