An air strike on a medical facility near the Syrian city of Aleppo has killed five employees of an international aid agency, the group says, including nurses.
There have been heavy air raids in and around Aleppo as the United Nations prepares to discuss the failed Syrian ceasefire.
The attack came a day after an aid convoy was targeted in a nearby raid.
Twenty people were killed in that attack, which the US blames on Russia.
The assault, on Monday, destroyed 18 lorries and has been described as a possible war crime.
Russia has strongly denied involvement of its own or Syrian planes, and said the incident was caused by fire on the ground and not by an air strike.
The Paris-based Union of Medical Care and Relief Organizations (UOSSM) said Tuesday's strike on its facility near Aleppo appeared to be targeted.
At least nine rebel fighters were also killed, the UK-based Syrian Observatory for Human Rights monitoring group said.
The two successive attacks were "not a coincidence," UOSSM's Dr Zaydoun al Zoubi told the BBC.
"Somebody is trying to tell us humanitarian workers are not welcome in Syria, that we are a target, that we will be killed," he said.
He called for the truce to be restored.
Separately, a Syrian warplane has crashed north of Damascus, the Syrian Observatory for Human Rights says. The so-called Islamic State group said the plane had been bombing its positions but did not claim they shot it down.
Buried under rubble
It is unclear who carried out the attack on the medical centre in the rebel-held town of Khan Touman late on Tuesday night local time.
The Syrian Observatory said either Syrian or Russian warplanes were responsible, Reuters news agency reports.
The strike hit a medical triage point, and killed two ambulance drivers and two nurses who had arrived to transport wounded patients to a more advanced medical facility, UOSSM said in a statement (in French).
A fifth medic is thought to have died later of their wounds.
The triage centre was completely destroyed. More victims are feared buried under the rubble.
The rebels killed were from Jaish al-Fatah, an Islamist group not officially part of the Western-backed alliance but one that works alongside the Free Syrian Army, the BBC's James Longman in Beirut reports.
Speaking earlier about Monday's attack on the aid convoy, unnamed US officials said two Russian SU-24 attack aircraft were in the sky above the convoy at the precise moment it was hit in Urum al-Kubra.
The strike, they added, was too sophisticated to have been carried out by the Syrian army.
White House spokesman Ben Rhodes later said: "There only could have been two entities responsible, either the Syrian regime or the Russian government.
"In any event, we hold the Russian government responsible for airstrikes in this space."
Russia's foreign ministry spokeswoman said the US government had "no facts" to support its claim.
"There are no craters and the exterior of the vehicles do not have the kind of damage consistent with blasts caused by bombs dropped from the air," a statement from the defence ministry said.
The UN had earlier said it was "not in a position to determine whether these were in fact air strikes".
Monday's attack prompted the UN to suspend all aid convoys in Syria.
The president of the International Committee of the Red Cross, Peter Maurer, denounced it as a "flagrant violation of international humanitarian law" and said it could amount to a war crime.
It occurred hours after the Syrian military declared the end of a week-long truce agreed by the US and Russia.