"The Arbin hospital was hit twice today and is now out of service," said Moussa Naffa, country director in Jordan for the Syrian American Medical Society (SAMS), which supported the clinic.

The news came as the United Nations said six hospitals had been hit in Eastern Ghouta in the past 48 hours, in addition to the one reported by SAMS.

The Syrian Observatory for Human Rights said the Arbin hospital was hit by a pair of Russian air strikes, in what would be the seventh hit in the past two days.

According to the Britain-based war monitor, it is the first time in three months that Russia has conducted raids on Eastern Ghouta, which it had designated last year as a "de-escalation" zone.

An AFP photographer in Arbin saw a Russian Sukhoi Su-34 fighter jet soaring overhead on Tuesday.

The United Nations said Tuesday that six hospitals have been struck in the past 48 hours in Eastern Ghouta, putting three out of service and killing several people.

"I am appalled and distressed by reports of the horrifying attacks against six hospitals in East Ghouta over the past 48 hours," said Panos Moumtzis, the UN's regional humanitarian coordinator for the Syria crisis.

At least 81 civilians were killed on Tuesday, including 15 civilians, in a fierce regime bombing campaign on the rebel-controlled enclave that began on Sunday.

The total civilian deaths since then, according to the Observatory, number at least 225 with hundreds more wounded.

Eastern Ghouta is the last rebel bastion on the capital's outskirts and Syrian President Bashar al-Assad appears more determined than ever to capture it.

An estimated 400,000 people live in the besieged region.