Military bombardment and shelling have continued overnight, killing at least eight people in the besieged Syrian enclave of Eastern Ghouta, despite the supposed ceasefire brokered by Russia, according to monitors.

The Syrian Observatory for Human Rights reported on Sunday that in the last 24 hours, the victims were killed following government bombardment across Eastern Ghouta, including in the city of Douma.

Government shelling and attacks were also reported in five other districts, including Harzma and Nashabiyah, it said.

Eastern Ghouta is the last remaining rebel stronghold near the capital, Damascus.

At least three missiles landed in the town of Irbin, the activist media monitor Ghouta Media Center (GMC) reported on Sunday, adding that it counted at least 15 attacks and shellings at the end of Saturday - the first day the truce was to take effect.

GMC reported that government troops were killed or injured, but did not specify a number, when they tried to enter a district in Eastern Ghouta controlled by the armed rebel group, Jaish al-Islam.

Eastern Ghouta has been under a government siege since 2013, and it is believed that 400,000 people still live in the area, which suffers from acute food and medicine shortages.

The ceasefire was negotiated in Vienna by the Russians.

"What is most concerning there is the situation for civilians," said Al Jazeera's Stefanie Dekker, reporting from the Syrian-Turkish border.

"Aid access into Eastern Ghouta for those people will be key, and will be something that then could really change and improve the situation for people on the ground.

"We know within this conflict, aid has always been a very difficult thing to negotiate."

UN heading to Sochi

Not all parties agreed to the Russia-backed ceasefire.

On Saturday, the armed Ahrar al-Sham group said it had rejected the ceasefire, adding it would never accept any deal involving the government of Syrian President Bashar al-Assad.

Over the past two months, Russian jets and the Syrian army have intensified their bombardment of the rebel enclave, killing more than 100 people, including at least four emergency volunteers from the White Helmets, according to the Syrian Observatory.

Amid the violence, diplomatic efforts continue to resolve the civil war, which has claimed at least 400,000 lives and displaced 22 million people since the civil conflict began in 2011.

On Saturday, the United Nations announced it will join Russia-sponsored talks in Sochi, despite a threatened boycott by the main Syrian opposition alliance.

Antonio Guterres' office said the UN secretary-general is sending Staffan de Mistura, UN special envoy for Syria, to take part in the Sochi meeting.

Guterres' office, however, stressed the talks should compliment UN-backed negotiations.

The two-day Sochi talks are expected to begin on January 29.

In announcing its boycott, the Syrian opposition said Russia is trying to sabotage the UN's effort to negotiate a deal.

"Geneva is the only place for reaching a political settlement [of the Syrian civil war]," Nasr al-Hariri, the chief negotiator for the opposition, told reporters on Saturday.