The United Nations is investigating attacks on hospitals and clinics in Syria that were carried out after information on their location was shared with Russia, the United Nation (UN) aid chief said today.

"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 emphasize how concerned I am about this."

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

Four health facilities -- two in Eastern Ghouta and two in northern Homs - were hit this year after their GPS coordinates were shared with Russia and the United States, according to the UN.

Russia co-chairs a UN humanitarian task force with the United States and also provided critical military backing to Syrian government forces in their offensive to retake Eastern Ghouta.

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

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

Lowcock told the council that a total of 92 attacks have been documented against Syrian health facilities and personnel in the first four months of 2018, killing 89 people and wounding 135.

Attacks on hospitals and medical facilities are a violation of international humanitarian law and have been repeatedly condemned by the Security Council.

Turning to humanitarian aid deliveries, Lowcock said the first aid convoy in more than two months is scheduled to arrive in northern rural Homs on Wednesday to deliver food and medicine to 93,000 people.

But aid deliveries to Douma, the main town in Ghouta that was recaptured by Syrian 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.

Now in its eight year, the war in Syria has killed more than 350,000 people and forced millions to flee. More than 13 million Syrians are in need of humanitarian aid, according to the UN.