At least 23 people, including five children, have been killed in a regime air strike in Syria's Eastern Ghouta, amid an intense period of bombing in the Damascus suburb.

Russian and Syrian government forces have launched a new wave of air strikes and shelling, plunging the besieged area just outside the country's capital into a spiral of violence and despair.

Around 60 civilians were killed on Tuesday in air strikes and shelling that overwhelmed rescue workers and prompted the United Nations to call call for an immediate cease-fire in order to deliver critical medical care to civilians across the country.

In Pictures: The crisis unfolding in Syria Show all 30 1 /30 In Pictures: The crisis unfolding in Syria In Pictures: The crisis unfolding in Syria A Syrian family arrives at a checkpoint, manned by pro-government forces, at the al-Hawoz street roundabout, after leaving Aleppo's eastern neighbourhoods Getty In Pictures: The crisis unfolding in Syria A Syrian woman, fleeing violence in the restive Bustan al-Qasr neighbourhood, reacts as she stands with her children in Aleppo's Fardos neighbourhood, after regime troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian pro-regime fighters, gesture as they drive past resident fleeing violence in the restive Bustan al-Qasr neighbourhood, in Aleppo's Fardos neighbourhood Getty In Pictures: The crisis unfolding in Syria Syrian rebels withdrew from six more neighbourhoods in their one-time bastion of east Aleppo in the face of advancing government troops, the Syrian Observatory for Human Rights said Getty In Pictures: The crisis unfolding in Syria Syrian rebels withdrew from six more neighbourhoods in their one-time bastion of east Aleppo in the face of advancing government troops, the Syrian Observatory for Human Rights said Getty In Pictures: The crisis unfolding in Syria Syrian residents, fleeing violence in the restive Bustan al-Qasr neighbourhood, arrive in Aleppo's Fardos neighbourhood , after regime troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian pro-regime fighters, gesture as they drive past residents fleeing violence in the restive Bustan al-Qasr neighbourhood, in Aleppo's Fardos neighbourhood Getty In Pictures: The crisis unfolding in Syria Syrian residents, fleeing violence in the restive Bustan al-Qasr neighbourhood, arrive in Aleppo's Fardos neighbourhood, after regime troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian residents, fleeing violence in the restive Bustan al-Qasr neighbourhood, arrive in Aleppo's Fardos neighbourhood Getty In Pictures: The crisis unfolding in Syria A Syrian pro-regime fighter speaks with a child, as residents flee violence in the restive Bustan al-Qasr neighbourhood. Syrian rebels withdrew from six more neighbourhoods in their one-time bastion of east Aleppo in the face of advancing government troops AFP/Getty Images In Pictures: The crisis unfolding in Syria Smoke rises as seen from a governement-held area of Aleppo, Syria Reuters In Pictures: The crisis unfolding in Syria Syrian soldiers targeting rebels-held areas in the eastern neighborhoods in Aleppo, Syria. According to media reports, the army is now holding on 99 percent of Aleppoís eastern neighborhoods EPA In Pictures: The crisis unfolding in Syria Syrian pro-government forces patrol Aleppo's eastern al-Salihin neighbourhood after troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian soldiers rest following the battle at al-Sheik Saeed neighborhood in Aleppo, Syria EPA In Pictures: The crisis unfolding in Syria A Syrian pro-government fighter walking past closed shops in the Bab al-Nasr district of Aleppo's Old City. Once renowned for its bustling souks, grand citadel and historic gates, Aleppo's Old City has been rendered virtually unrecognisable by some of the worst violence of Syria's war Getty In Pictures: The crisis unfolding in Syria The crucial battle for Aleppo entered its 'final phase' after Syrian rebels retreated into a small pocket of their former bastion in the face of new army advances. The retreat leaves opposition fighters confined to just a handful of neighbourhoods in southeast Aleppo, the largest of them Sukkari and Mashhad Getty In Pictures: The crisis unfolding in Syria Syrian civilans arrive at a checkpoint, manned by pro-government forces, at the al-Hawoz street roundabout, after leaving Aleppo's eastern neighbourhoods. Syria's government has retaken at least 85 percent of east Aleppo, which fell to rebels in 2012, since beginning its operation Getty In Pictures: The crisis unfolding in Syria Syrian civilians flee the Sukkari neighbourhood towards safer rebel-held areas in southeastern Aleppo Getty In Pictures: The crisis unfolding in Syria Syrians celebrate in the government-held Mogambo neighbourhood of the northern Syrian city of Aleppo, after rebel fighters retreated into a small pocket of their former bastion in the face of new army advances Getty In Pictures: The crisis unfolding in Syria Syrians celebrate in the government-held Mogambo neighbourhood of the northern Syrian city of Aleppo, after rebel fighters retreated into a small pocket of their former bastion in the face of new army advances. The fall of Aleppo would be the worst rebel defeat since Syria's conflict began in 2011, and leave the government in control of the country's five major cities Getty In Pictures: The crisis unfolding in Syria A Syrian refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria Syrian refugee Aliya inside the tent where she lives with her husband and ten children in a camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria Syrian refugee women and children outside the entrance to their tents in the refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA Wire In Pictures: The crisis unfolding in Syria A Syrian refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria A Syrian refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA Wire In Pictures: The crisis unfolding in Syria A Syrian refugee woman outside the entrance to the tent where her family live, in the refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria A vehicle drives past a mosque at night in Idlib, Syria. Picture taken with a long exposure Reuters In Pictures: The crisis unfolding in Syria Damaged buildings stand in the rebel-controlled town of Binnish in Idlib province, Syria Reuters In Pictures: The crisis unfolding in Syria The night sky is seen through damaged windows in the rebel-controlled town of Binnish in Idlib province, Syria Reuters In Pictures: The crisis unfolding in Syria Damaged buildings stand in the rebel-controlled area of Maaret al-Numan in Idlib province, Syria Reuters

The punishing campaign by the Syrian government and its ally has also battered the northwestern province of Idlib, the largest area under opposition control in Syria.

The Idlib offensive hit hospitals and residential buildings and has intensified after militants shot down a Russian Su-25 fighter jet near the town of Saraqeb over the weekend.

Video later surfaced which is said to show the final moments of the pilot as he detonated a grenade to avoid being captured by fighters from Hayet Tahrir al-Sham, an al-Qaeda affiliated group.

Video purports to show final moments of Russian pilot shot down in Syria

The UN mission in Damascus warned of "dire consequences" to the humanitarian crises in Syria, identifying seven areas needing urgent relief.

The ongoing fighting and deliberate obstructions by forces were preventing aid organisations from reaching civilians, it said.

A Syrian man looks at the rubble following air strikes on the rebel-held besieged town of Douma in the Eastern Ghouta region, on the outskirts of the capital Damascus (HAMZA AL-AJWEH/AFP/Getty Images) (Getty)

The lead investigator of a UN-mandated Commission of Inquiry on Syria said his team was looking into the reports bombs allegedly containing weaponised chlorine were used on two recent occasions in the town of Saraqeb, in Idlib, and Douma, in Eastern Ghouta.

In a statement, Paulo Pinheiro said the spiraling violence had made "a mockery of the so-called 'de-escalation zones"' — an agreement reached last year among Russia, Iran and Turkey to stabilise the lines of conflict and open corridors for humanitarian aid.

He described the government's siege and indiscriminate bombardment of eastern Ghouta as "international crimes."

On Wednesday, France's foreign minister said "all indications show us today that the Syrian regime is using chlorine gas at the moment."

Jean-Yves Le Drian added: "An investigation has been opened on that matter by the United Nations.

"The threat of using chemical weapons remains, this is a very serious situation."

Meanwhile, Syria said it had responded to "a new Israeli aggression" on Wednesday, after aircraft targeted a military outpost near Damascus.

State news agency SANA said air defences confronted the Israeli planes, which it said were flying over Lebanese territory when they fired the missiles and the army claimed its air defence system destroyed most of the missiles.

"Our air defence systems responded to missiles fired by the Israeli enemy air force from over Lebanese territory on a military outpost in Damascus countryside and destroyed most of them," an army statement said.

The Syrian Observatory for Human Rights said the air strike targeted positions of the Syrian army and its allies in the area of Jamraya, northwest of Damascus, which is home to a government research centre and was also hit in December.

Syria's multisided war has drawn in armies and insurgents from around the world, aggravating a conflict that began in 2011 as an uprising against its President, Bashar al-Assad.

Though he appears to have survived the insurgency, his forces continue to bomb and besiege opposition pockets across Syria, including in Eastern Ghouta and Idlib.