A Saudi military helicopter that crashed in Yemen killing 12 Saudi soldiers was likely hit by "friendly fire," military sources said Wednesday.

In a sign of the growing regional influence of Iran's Revolutionary Guards, a close adviser to its foreign operations commander began his new role as ambassador to Iraq on Wednesday, state media reported.

The United States wants to see a strong Saudi Arabia, U.S. Defense Secretary Jim Mattis said during talks Wednesday aimed at reinvigorating the Riyadh-Washington alliance.

The head of a global arms watchdog said Wednesday that "incontrovertible" test results from an alleged chemical strike in Syria showed sarin gas or a similar substance had been used.

Samples from 10 victims of the April 4 attack on Khan Sheikhun analyzed at four laboratories "indicate exposure to sarin or a sarin-like substance," said Ahmet Uzumcu, head of the Organization for the Prohibition of Chemical Weapons.

Palestinian leaders on Wednesday denounced Israel's refusal to negotiate with Palestinians on hunger strike in Israeli jails, warning of a "new intifada" if any of them die.

A bomb killed six people and wounded more than 30 in Syria's second city Aleppo on Wednesday, state television reported. 

The blast hit the southwestern neighbourhood of Salaheddin, which was once on the front line between rebels and government forces before the army retook full control of the city in December. 

Two strikes apparently carried out by US drones killed five suspected al-Qaida members in Yemen overnight, military sources said on Wednesday.

Evacuations of thousands of besieged Syrians resumed on Wednesday with tight security in place after a weekend bombing against those leaving government-held areas killed dozens, including nearly 70 children. 

A large convoy of buses set out from the government-held towns of Fuaa and Kafraya in Idlib province, carrying 3,000 people to the rebel-held transit point of Rashidin near Aleppo, an AFP correspondent said.

A policeman was killed and three others wounded Tuesday when gunmen opened fire on a checkpoint near St Catherine's monastery in Egypt's Sinai, the interior ministry said, in an attack claimed by Islamic State jihadists.

"A number of gunmen... facing one of the checkpoints on the road to St Catherine's in south Sinai shot at security forces at the checkpoint," the interior ministry said in a statement on its Facebook page.

Egyptian police on Tuesday arrested a man wanted for alleged involvement in twin church bombings this month claimed by the Islamic State group, an official said.