The United Nations is investigating attacks on hospitals and clinics in Syria carried out after their locations were disclosed to Russia, the UN aid chief says.

"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 UN Security Council on May 29.

Lowcock did not provide details but emphasized his concern, saying "It is an issue on which I may have to come back to you."

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.

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 says.

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 during 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.

Based on reporting by AFP, AFP, and Reuters