An air strike in northern Syria killed five medics responding to an earlier bombing raid, a relief group said, a day after an air strike on a humanitarian convoy prompted the UN to suspend desperately needed aid deliveries.

The team had just arrived at the scene of the first air strike in the rebel-held town of Khan Touman when planes circled around and struck the area again, D. Oubaida Al Moufti, vice president of the International Union of Medical Care and Relief Organisations, said.

Syrian government forces have been accused of carrying out "double tap" attacks throughout the five-and-a-half-year war, placing paramedics and rescue workers in peril.

The organisation, known by its French initials UOSSM, had initially said that the Tuesday night strike levelled a medical triage point it operates in rebel-held territory outside the contested city of Aleppo.

The Britain-based Syrian Observatory for Human Rights said at least 13 people were killed in the attack, including nine militants, some of them belonging to the Fatah al-Sham Front, an al Qaida-linked group previously known as the Nusra Front.

Three nurses and two ambulance drivers died of their injuries, UOSSM said.

It was not immediately clear who carried out the strike. Aircraft from Syria, Russia and the US-led coalition are targeting the Fatah al-Sham Front, which along with the Islamic State group was excluded from a week-long ceasefire.

An air strike on a Syrian Arab Red Crescent aid convoy on Monday night prompted international condemnation over attacks targeting humanitarian facilities and workers.

UN secretary-general Ban Ki-moon described the convoy strike as a "sickening, savage and apparently deliberate attack". The convoy was carrying UN aid.

The incident fuelled tensions between the two architects of Syria's ceasefire, Russia and the United States.

Washington said it believed Russian or Syrian government jets were behind the attack that killed 20 civilians, and that either way it held Russia responsible because under the truce deal Moscow was charged with preventing air strikes on humanitarian deliveries. Syria's rebels do not have aircraft.

In New York on Tuesday, Russian and US diplomats insisted that the ceasefire, which went into effect nine days ago, was not dead, despite soaring violence.

The Syrian military said the truce expired on Monday night, shortly before presumed Russian or Syrian government jets launched a sustained aerial attack on Aleppo's opposition-held neighbourhoods.

The ceasefire was intended in part to allow humanitarian convoys to reach besieged and hard-to-reach areas throughout Syria. Yet following the convoy attack, the UN suspended overland aid operations in Syria. Syrians living in opposition areas will be disproportionately affected because the UN's major warehouses are located in government-held areas.

The UN estimates that some six million Syrians live in besieged and hard-to-reach areas.

US Secretary of State John Kerry wants all aircraft over key humanitarian routes in northern Syria grounded in order to facilitate desperately need aid deliveries.

Mr Kerry told the UN Security Council on Wednesday that such a step could restore credibility to efforts to end the five-year civil war and "give a chance for humanitarian assistance to flow unimpeded".

Elsewhere in Syria, air strikes on the opposition-run town of Talbiseh, outside of the central city of Homs, killed two civilians and wounded tens of others, according to the activist-run Talbiseh Media Centre.

The town is besieged by government forces. The Observatory said government jets subjected the town to heavy bombardment on Wednesday morning.

State TV said a Syrian war plane crashed after carrying out a combat mission against Islamic State militants north-west of the capital, Damascus. The pilot was rescued, according to an unnamed military official quoted on state TV.

The IS-affiliated news agency Aamaq said the group downed the plane in the eastern Qalamoun mountains after the aircraft carried out four raids. IS shot down a government aircraft on Sunday in the eastern Deir el-Zour province.