AMMAN/UNITED NATIONS: The Syrian army stepped up shelling and airstrikes on the last rebel-held enclaves in the Syrian capital Monday, its heaviest bombardment in a two-month campaign, rebels and witnesses said.

From the strategic Qasioun Heights that overlook Damascus, elite units of the army pounded Jobar district, some 2 km east of the Old City wall, and Ain Tarma just to the south.

The army’s offensive has dented a Russian-sponsored cease-fire announced two weeks ago in the Eastern Ghouta area to the east of Damascus. Moscow has said it is negotiating with mainstream rebel groups in several areas across Syria to create new de-escalation zones and calm fighting.

Rebels and residents said Moscow had already begun deploying some military police in certain checkpoints that border the rebel-held northern Homs countryside and in southwestern Syria where de-escalation zones have been announced.

Residents and rescuers say the Syrian army has intensified shelling of civilian areas in the Eastern Ghouta region where rebels are holed up, many in underground tunnels, out of frustration at the lack of progress.

At least 15 civilians have been killed and scores of others injured in three days of sustained bombardment. The U.K. based Syrian Observatory for Human Rights, which monitors the conflict, said at least 20 army soldiers were either killed, injured or taken prisoner in the past 24 hours, confirming rebel estimates.

Less intense shelling has hit Zamalka, Harasta and Kafr Btna, also in Eastern Ghouta. Social media videos downloaded by activists showed sorties of jets pounding areas in Ain Tarma and Jobar, with fire erupting in places and large plumes of smoke rising into the air.

Eastern Ghouta has been under opposition control for much of the 6-year-old conflict. Jobar is in northeast Damascus and borders the Eastern Ghouta district of Ain Tarma.

The army is also using more elephant rockets – inaccurate and improvised munitions often made from gas canisters and fired on a high trajectory – insurgents said.

“The elephant rockets are not having mercy on us. We have dug tunnels and fortified our positions so they are unable to advance,” said Abu Obada al-Shami, a commander from Failaq al-Rahman, the rebel group whose fighters are drawn from the area.

The rebels, for their part, are trying to hang on to their last foothold in Damascus following the loss earlier this year of Qaboun and Barzeh districts, located north of Jobar.

Before the war began in 2011, more than half a million people lived in Eastern Ghouta, a sprawling mix of towns and farmland.

Two residents said Ain Tarma was now a ghost town, with only a few hundred families taking shelter in basements after most former residents fled to other towns in Eastern Ghouta. “Life is nonexistent. Permanent terror and people are not coming out of their basements,” said Abdullah al-Khatib, a former electrician who lives there with his eight-member family.

Rebels accuse the Syrian army and its Iran-backed allies of breaking the Russia-brokered truce in Eastern Ghouta to throw its full weight against Jobar and Ain Tarma. The government has said it would abide by the truces which Russia has brokered, but says it continues to target Islamist militant factions not covered by the agreement.

Meanwhile, the U.N. says Secretary-General Antonio Guterres is expressing regret at the resignation of a prominent member of the Independent Commission of Inquiry on Syria and is stressing the need for accountability for crimes against civilians during the war.

U.N. spokesperson Stephane Dujarric told reporters Monday that Guterres believes accountability is “critical” and the commission’s work is “an important and integral part of the accountability process.”

Carla del Ponte said in comments published Sunday by the Swiss magazine Blick that she was resigning in frustration at the Security Council’s inaction to hold Syrian criminals accountable.

Dujarric said Guterres is “grateful for her service” to the commission and “as a tireless advocate for the cause of accountability throughout her career.”