Rebel groups in Syria‘s north say they have received their largest shipment of weapons yet, in a fillip to an anti-government campaign that had stalled for many months.

Leaders of militias supported by backers in Saudi Arabia and Qatar say several hundred tonnes of ammunition and a limited supply of light weapons were allowed across the Turkish border in the past three days, in what they said was the first large-scale re-supply since earlier this year.

The weapons are believed to have been sent by Saudi Arabia and Qatar and were warehoused in Turkey for many months. Senior rebel commanders contacted by the Guardian say they did not include anti-aircraft missiles, but several dozen anti-tank rockets were among them.

The delivery came after an apparent chemical weapons in eastern Damascus on Wednesday, the site of which will be visited by UN investigators. It is not yet clear whether the widespread scenes of horror, in which many hundreds of people were killed, influenced the decision.

“For months we have not been able to advance along a front,” said a commander of the Salafist militia, Ahrar al-Sham, who did not want to be named. “This will allow us to fight more like an army.”

Armed opposition ranks in northern Syria are a mix of militias trying to replace Syrian president Bashar al-Assad as leader and jihadist groups who see Syria as an essential arena in a global struggle to install hardline Islamic rule and turn the country into the seat of a new caliphate.

Jihadists, among them thousands of non-Syrians, have gained increasing prominence in the north and keeping weapons supplies away from them has been a key demand of the US and Europe, which fears that warlords hold sway across sizable chunks of the country.

One such jihadist group, the al-Qaida associated Jabhat al-Nusra, warned on Sunday that it would seek to avenge the chemical weapons attack on the eastern Ghouta region near Damascus by bombing communities of Alawites — the Shia sect to which Assad belongs.

In an audio recording posted on a militant website, Abu Mohammed al-Golani addressed families of children killed in the attack: “The revenge for the blood of your children is a debt to be paid back … 1,000 rockets will be fired at them in revenge for the massacre of Ghouta.”

Other rebel factions in Damascus have also threatened to mount reprisal attacks. Opposition groups universally blame the Assad regime for the strike, which has since been viewed as the worst chemical weapons attack since Saddam Hussein killed between 3,000 and 5,000 Kurds in Iraqi Kurdistan in 1988.

Meanwhile, survivors from east Ghouta continue to trickle across the borders of Jordan and Lebanon, where intelligence agencies have taken biological samples from them to send to Europe and the US.

The three areas of east Ghouta where the attack took place have been heavily bombed by conventional weapons in the past four days, making passage out of the area even more difficult than normal. However, rebel groups say they have secured smuggling lines to Jordan in particular.

“Lebanon is more difficult, not just to get there, but to move around once they arrive,” said an east Ghouta rebel leader, Abu Khalil. “The health ministry is controlled by the Amal movement, which reports to Hezbollah [the Lebanese-headquartered Shia militia whose troops are fighting alongside Assad’s forces]. Dealing with them is a big risk.”

Turkey is considered by far the safest of Syria’s four borders for rebels to operate, but it is a long and perilous seven day journey from Damascus along roads that are readily bombed by the Syrian air force.

Saudi Arabia, which opposes the Assad regime, had this year been using the Jordanian border to smuggle weapons into Syria, after complaining that the US was allowing few supplies to cross from Turkey.

Groups in southern Syria have appeared in videos boasting new heavy weapons that were sourced from Croatia. They had proved decisive in some battles in the south, but some had later turned up in the hands of jihadist groups in the east of the country.

“No matter how hard we try, it is always going to be an arms bazaar in these lawless areas,” said a western official in the region. “There is barter and trade going on all the time and that is a price we have to pay if we decide to go for broke on these weapons.

“My guess is that east Ghouta has made some people drop their guard for a moment, but the primary threat remains — and that is al-Qaida who are strong and getting stronger. We are determined not to contribute to that.”