More than 250 people have reportedly died after two days of intense bombardment from the Syrian government on the rebel enclave of eastern Ghouta near Damascus. At least 50 children are believed to have perished in the shelling, with 1,000 maimed and wounded.

Opposition activists said government forces have brought in more reinforcements in recent days, suggesting a major assault is imminent to recapture the last main rebel stronghold near Damascus.

The eastern Ghouta suburbs are home to some 400,000 people as well as thousands of insurgents belonging to different factions. The most powerful is the ultra conservative Army of Islam and Failaq al-Rahman, with a small presence of al-Qaida-linked fighters.

Videos showed paramedics pulling out the injured from under the rubble while others are seen frantically digging through the debris in the dark, in search of survivors.

Ahmad al-Dbis, an official at the Union of Medical Care and Relief Organizations, which runs hospitals and clinics in Syrian opposition areas, says ten hospitals have been damaged by airstrikes or shelling in the last 48 hours.

"Yesterday we had more than a hundred martyrs. More than 600 wounded. Today, now we are in the midday, we have more than 80 martyrs," a doctor at one of the hospitals told ITV News.

"From this morning, this early morning, continuous shelling, airstrikes is going on. So civilians in eastern Ghouta are frightened. It's very hard life here, its a miserable life. We want the world to demonstrate credibility about human rights. We want the the world to stop this crime against civilians in eastern Ghouta."

The government bombardment - backed by Russian forces - has seen the use of warplanes, helicopter gunships and missiles as well as artillery in a major escalation of violence near Bashar al-Assad's seat of power.

The Syrian Observatory for Human Rights said it was the deadliest days in eastern Ghouta since 2015.

The opposition-affiliated Syrian Civil Defence, also known as White Helmets, said many victims are still under the rubble.

Both the Observatory and the White Helmets reported more air strikes and shelling on Tuesday in eastern Ghouta as rebels pounded Damascus with mortar shells.

The Observatory said the "crazy shelling" appears to be paving the way for a wide ground offensive on eastern Ghouta. If captured by government forces it would be another major victory for Assad since the conflict began seven years ago.

Rebels retaliated by hitting some Damascus neighbourhoods with mortar shells, killing eight, including three children, and wounding 15 people, according to the state news agency SANA. On Tuesday morning, Damascus residents reported shelling of areas in the centre.

The UN envoy for Syria, Staffan de Mistura, decried a recent upsurge in violence, expressing concerns that eastern Ghouta could fall victim to widespread bloodshed like that in northern Aleppo more than a year ago.

The UN children's agency, Unicef, issued a one-page statement of protest against the killings and carried a headline, saying: "Do those inflicting the suffering still have words to justify their barbaric acts?"

The International Committee of the Red Cross also issued a statement saying that "this cannot go on". It added that there had been "distressing reports" of dozens injured and killed every day in eastern Ghouta, with "families trapped, with no safe place to hide from shelling. Dozens of mortars in Damascus cause civilian casualties and spread fear. We cannot let history repeat itself."