UN Secretary-General Antonio Guterres calls for end to 'hell on earth' in Syria's eastern Ghouta

United Nations Secretary-General Antonio Guterres called on Monday for warring sides to implement a 30-day ceasefire across Syria, in line with an appeal by major powers at the weekend.

UN aid agencies are ready to deliver life-saving aid and evacuate critically wounded from the rebel-held Damascus enclave of eastern Ghouta, where 400,000 people have been living under siege, Guterres said.

"Eastern Ghouta cannot wait, it is high time to stop this hell on earth," Guterres told the UN Human Rights Council, which opened its main four-week annual session in Geneva.

UN human rights chief Zeid Ra'ad al-Hussein said air strikes on eastern Ghouta were continuing on Monday morning.

The UN Security Council on Saturday demanded a 30-day truce across Syria as rescuers in the country's eastern Ghouta region said bombing had not let up long enough for them to count bodies during one of the  bloodiest air assaults of the seven-year war.

Guterres appealed on Wednesday for an immediate end to "war activities" in eastern Ghouta, where nearly 400,000 people have lived under government siege since 2013, without enough food or medicine.

While Syrian ally Russia supported the adoption of the UN resolution, Russian UN Ambassador Vassily Nebenzia cast doubt on its feasibility. Previous ceasefire deals on the ground have had a poor record of ending fighting in Syria, where President Bashar al-Assad's military has gained the upper hand.

After several days of delay and last-minute negotiations to win the support of Russia, the council adopted the resolution - drafted by Sweden and Kuwait - demanding hostilities cease for 30-days "without delay" to allow aid access and medical evacuations.

A surge of rocket fire, shelling and air strikes has killed more than 500 people since Sunday night, the Syrian Observatory for Human Rights said. The dead included more than 120 children.

The monitor said raids hit Douma, Zamalka and other towns there on Saturday, killing 40 people.

After the UN vote the two dominant rebel factions in Ghouta - Failaq al-Rahman and Jaish al-Islam - both committed to implement the truce and facilitate aid access, but also reiterated their right to respond to any attacks on them.

Medical charities have decried attacks on a dozen hospitals. The Syrian government and Russia say they only target militants. Moscow and Damascus have said they seek to stop mortar attacks injuring dozens in the capital, and have accused insurgents in Ghouta of holding people as human shields.

Syrian state media said Ghouta factions fired mortars at districts of Damascus on Saturday, including near a school. Insurgent shelling wounded six people, it said, and the army heavily pounded militant targets in response.

The Ghouta pocket has become the war's latest flashpoint, after a string of rebel defeats and negotiated withdrawals. With Russian jets and Iran-backed militias, Assad's military has restored state rule over the main cities across western Syria.

Insurgents in eastern Ghouta have vowed not to accept such a fate, ruling out the kind of evacuation that ended rebellion in Aleppo and Homs after bitter sieges.

Russia has blamed Nusra fighters, from al-Qaeda's former Syria branch, for provoking the situation in Ghouta. The two main Islamist factions there in turn accuse their enemies of using the presence of a few hundred jihadist fighters as a pretext for attacks.