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"We are investigating a number of cases of medical facilities being attacked shortly after having been deconflicted," Mark Lowcock, the UN under-secretary-general for humanitarian affairs, told the Security Council.

Lowcock did not provide details but said: "I want to emphasise how concerned I am about this."

"It is an issue on which I may have to come back to you," he told the council.

The GPS coordinates of 661 health facilities had been shared since the start of this year with Russia and the US.

In four specific incidents, an attack took place despite the notification placed – two in Eastern Ghouta, and two in northern Homs, according to the UN.

The two sites hit in Ghouta were a hospital in the town of Arbin late March, and a children's hospital in Douma early April said Panos Moumtzis, the UN regional coordinator for Syria.

In Homs, two facilities in the town of Zafraniyeh were hit in late April.

A total of 92 attacks have been documented by the UN against Syrian health facilities and personnel in the first four months of 2018, killing 89 people and wounding 135.

Almost half of this year's attacks took place in Ghouta, where the Syrian regime, backed by Russia, launched a huge offensive on the opposition enclave.

The Security Council has repeatedly condemned attacks on hospitals and medical facilities, which are a violation of international humanitarian law.

The first aid convoy in two months is expected to arrive in northern rural Homs on Wednesday, said Lowcock, where food and medicine need to be delivered to 93,000 people.

Aid deliveries to Douma however, the main city in Ghouta captured by Syrian regime forces in April, continue to be blocked.

Lowcock urged the Syrian government to allow access to Eastern Ghouta where more than 10,000 people have returned in the past two weeks and almost 200,000 have remained throughout the recent fighting.

The UN has previously slammed Syria's escalation against the health facilities as "shocking", saying that more facilities were attacked in the first four months of 2018 than all of last year combined.

Syria today accounts for nearly 70 percent of all reported attacks on health facilities worldwide.

More than 500,000 people have been killed and millions displaced since Syria's war started in 2011 with the brutal repression of anti-government protests.

Russia and the Syrian regime have been blamed by human rights groups and NGOs for routinely hitting hospitals, as well as other civilian areas.