Gulf-based supporters have sent a 400-tonne shipment of arms to Syria's outgunned rebels, one of the biggest to reach them in their two-year-old uprising, opposition sources said on Sunday.

The consignment - mostly ammunition for shoulder-fired weapons and anti-aircraft machine guns - came into northern Syria via the Turkish province of Hatay in the past 24 hours, and was already being handed out, the sources added.

One rebel officer told Reuters the flow of arms bound for rebels had increased since opposition groups accused the government of launching deadly chemical weapons attacks in Damascus on Wednesday.

"Twenty trailers crossed from Turkey and are being distributed to arms depots for several brigades across the north," said rebel official Mohammad Salam, who told Reuters he saw the weapons come over the border.

Syria's conflict, pitting mostly Sunni Muslim insurgents against President Bashar al-Assad, whose Alawite sect follows an offshoot of Shi'ite Islam, has ignited sectarian tensions across the region.

Qatar and other Sunni-led Gulf states have backed the insurgents, while Shi'ite power Iran remains one of Assad's main allies.

Rebel units operating in northern Syria range from moderate Islamists to hardline jihadists and include Liwa al-Islam (The Division of Islam), Sukur al-Sham (The Hawks of Syria), The Free Martyrs of Syria, Ahfad al-Rasul (The Grandsons of the Prophet) and Ahrar al-Sham (The Freemen of Syria).

Analysts say they have struggled to hold onto advances against Assad's Iranian and Russian-armed forces and his monopoly on air power.

A senior officer in the Gulf and Western-backed Supreme Military Council, an umbrella group for rebel units, said there had been an increase in rebel-bound arms shipments coming into Turkey, particularly since the reports of a chemical assault.

The government has dismissed Syrian opposition accusations that it killed well over 1,000 civilians with poison gas in Damascus suburbs on Wednesday.

Weapons still waiting to cross into Syria included more sophisticated anti-tank guided weapons, the officer added, without elaborating.

He, and other sources, said money for the shipments came from the Gulf, without naming countries.

Western diplomats fear some of the increasing flow of Gulf-financed weapons could end up in the hands of groups linked to al Qaeda. – Reuters