The US and UK have suspended all "non-lethal" support for rebels in northern Syria, but not humanitarian aid.
A US spokesman said it was concerned about reports that Islamist rebels had seized bases belonging to the Western-backed Free Syrian Army (FSA).
Fighters from the Islamic Front, a new alliance of rebel groups, ousted FSA-aligned fighters from the Bab al-Hawa border crossing with Turkey last week.
The non-lethal aid includes medicine, vehicles and communications equipment.
The US and European countries have been reluctant to supply weapons and ammunition directly to rebel groups in Syria because of concerns that they might end up in the possession of jihadists affiliated to al-Qaeda.
However, they have reportedly facilitated secret arms shipments.
White House spokesman Josh Earnest confirmed the US had "suspended all further deliveries of non-lethal assistance into northern Syria" as a result of events at Bab al-Hawa.
But he stressed that humanitarian aid was not affected by the decision.
UK Foreign Office Minister Hugh Robertson told the BBC that "as far as we know at the moment" no British equipment had passed into the hands of Islamist militants, but he added: "It does make sense to suspend that aid until we know exactly what's happened."
The FSA said the suspension was a mistake. "We hope our friends will rethink and wait for a few days when things will be clearer," spokesman Louay Meqdad was quoted by Reuters news agency as saying.
Last month, seven leading rebel groups - the Ahrar al-Sham, Jaysh al-Islam, Suqour al-Sham, Liwa al-Tawhid, Liwa al-Haqq, Ansar al-Sham and the Kurdish Islamic Front - declared that they were forming the largest alliance yet in the 33-month conflict, with an estimated 45,000 fighters.
They said the new Islamic Front was an "independent political, military and social formation" that aimed to topple President Bashar al-Assad's government and build an Islamic state.
The front does not include al-Qaeda affiliates like the Islamic State in Iraq and the Levant (ISIS) and the al-Nusra Front, but its charter welcomes "muhajirin", or foreign fighters, as "brothers who supported us in jihad", and suggests it is willing to co-operate with them.
Last week, the Islamic Front announced that it had withdrawn from the command of the FSA's Supreme Military Council (SMC), which is aligned to the opposition National Coalition.
Four days later, its fighters drove out SMC-aligned forces out of their bases and warehouses at Bab al-Hawa, in the north-western province of Idlib, which contained weapons and equipment that had been brought into Syria through Turkey.
The Syrian Observatory for Human Rights, a UK-based activist group, said dozens of anti-aircraft weapons and anti-tank rockets were taken.
Mr Meqdad said the Islamic Front had raised its flag in place of the SMC's after "asking" its personnel to leave. But he also stressed: "We believe that those brigades are our brothers, that they know that we are not the enemy."
On Wednesday, the US embassy spokesman told the Reuters news agency that the situation at Bab al-Hawa was being investigated to "inventory the status of US equipment and supplies provided to the SMC".
Humanitarian assistance by the US would not be affected because it was distributed through international and non-governmental organisations, the spokesman added.
Officials in Washington told the Associated Press that deliveries of non-lethal aid to the south of Syria via Jordan would also continue.
The US government has committed to provide $250m (£152m) in non-lethal assistance to the National Coalition, local opposition councils and the SMC. Rebel brigades have been provided with food rations, medical supplies, communications equipment and vehicles.
The UK is providing more than £20m in non-lethal support, including 4x4 vehicles, body armour, generators, communications equipment, water purification kits and equipment to protect against chemical weapons.
The rebel infighting comes as government forces make advances.
In the past two months, several towns around the capital Damascus and the second city of Aleppo have been recaptured by soldiers backed by pro-government militiamen, members of the Lebanese Shia Islamist movement Hezbollah and Iranian Revolutionary Guards, activists say.
An offensive is also under way in the Qalamoun mountains, which run along the border with Lebanon. On Tuesday, troops started moving towards the town of Yabrud, the last rebel stronghold in the area.