UPDATE: 3:51 a.m. EDT -- The U.K.-based Syrian Observatory for Human Rights said Thursday that warplanes pounded the rebel-held areas of Syria’s Aleppo city overnight in what the monitor describes as the heaviest airstrikes in months. The Syrian military is yet to comment on the strikes, reports said.

Original story:

A fresh airstrike in northern Syria killed four medics belonging to an international aid agency, the relief group said Wednesday. The airstrike, said to have occurred Tuesday, follows Monday’s airstrike that targeted aid trucks from the United Nations and killed 20 people.

The violence against aid workers added to the failure of the U.S.-Russia brokered ceasefire proposed earlier this month. Tuesday’s attack took place shortly after aid workers arrived in Syria’s Khan Touman town near Aleppo following an earlier bombing. 

Planes circled around and struck the area again as aid workers set out to help the affected, Oubaida Al Moufti, vice president of the Union of Medical Care and Relief Organizations (UOSSM), reportedly said. The group later added that in addition to the five medics who were killed in the airstrike, two nurses and two ambulance drivers were also killed while a third nurse is in a critical condition.

The Paris-based organization added that the airstrike at its facility appears to be targeted. UOSSM’s Zaydoun al Zoubi told the BBC that the two attacks were “not a coincidence.”

“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.

The news of the airstrike comes just as the U.N. announced Wednesday that it would resume delivery of aid to the war-torn country as early as Thursday. “Our obligation to civilians on all sides is to go where and when we can with relief,” Jan Egeland, a senior U.N. humanitarian official focused on Syria, reportedly said. “We hope to resume convoys tomorrow and Friday, but still work on security guarantees.”

Monday’s attack had already added to the mounting tension between Russia and the U.S. who blamed each other for the same. U.S. Secretary of State John Kerry said Tuesday at the U.N. Security Council session that Russia appears to be operating “in a parallel universe” when it comes to Syria.

“There only could have been two entities responsible, either the Syrian regime or the Russian government,” Deputy U.S. National Security Adviser Ben Rhodes said Tuesday. “In any event, we hold the Russian government responsible for airstrikes in this space.”

Russia, on the other hand, vehemently denied the accusation with the country’s foreign ministry spokeswoman insisting that Russia had “nothing to do with this situation.”

The proposed ceasefire was to last one week after which the U.S. and Russia would form a military partnership to tackle the Islamic State group and the Syrian Conquest Front, a former affiliate of al Qaeda that was earlier called the Nusra Front. However, the deal crumbled a few days later following multiple violations, including a U.S.-led airstrike Saturday that reportedly resulted in the deaths of over 60 Syrian soldiers.