Editor's Note: This story contains graphic images of injured children.
At least 27 people were killed and 60 more injured Wednesday in Syrian government air strikes on three towns in rebel-held Eastern Ghouta, near Damascus, the Syrian Observatory for Human Rights said.
Twelve children were among the dead, the UK-based monitoring group said.
It's the third day running the besieged area, to the east of the Syrian capital, has been targeted by Russian-backed Syrian forces.
At least 80 people -- including 19 children and 20 women -- were killed and 160 injured in a heavy bombardment on Tuesday, SOHR said.
That made it the deadliest day in the Syrian conflict since 83 people were killed in an attack on a market in the town of al-Atareb, in the western countryside of Aleppo, nearly two months ago, according to the monitoring group.
The rebel-held Eastern Ghouta area has been surrounded by Syrian government forces for more than four years. The siege intensified last May after government forces conducted a large-scale offensive in the area.
Airstrikes by Russian-backed Syrian government forces have escalated in recent days following the shooting down of a Russian warplane in the rebel-held northwestern province of Idlib over the weekend.
According to Syrian activists, conditions under the siege in Eastern Ghouta are the worst they have been in four years and continue to deteriorate, with hundreds of thousands of people suffering a lack of food and medical supplies.
The situation has worsened since most makeshift tunnels used to smuggle food and other essential materials from Damascus were found and blocked by Syrian regime forces.
UN officials expressed "deep concern" this week over the escalation of violence in Eastern Ghouta and Idlib.
Human rights experts are investigating reports of alleged chlorine bomb attacks on civilians in two Syrian towns, Douma in Eastern Ghouta and Saraqeb in Idlib, the UN's commission of inquiry on Syria said.
The Organisation for the Prohibition of Chemical Weapons said Wednesday that recent allegations of chemical weapons use were of "grave concern" and that its fact-finding mission in Syria continued to investigate.
Syria: Army responding to attack
A video shared by the rebel-affiliated Ghouta Media Center on Wednesday shows an unnamed old man standing next to two children he claims were killed in recent attacks on Eastern Ghouta.
"We call on the United Nations and Arab Muslim countries to come see what the Syrian regime is doing in our towns and homes. The Syrian regime claims to kill terrorists -- are those terrorists?" he asks. "They are killing innocent people, women and children, in cold blood. They are not killing Nusra Front fighters or ISIS or any other militant group, but they are killing civilians," he says.
Syrian state-run media said Tuesday that the Syrian army was responding to a shelling attack in Damascus by "armed groups" -- an attack that they say killed five people, including two children, and injured 13 others.
Regime forces targeted "areas from which the shells were launched in Eastern Ghouta, inflicting heavy losses upon the armed groups' ranks and destroying a number of their mortar launchers," the state-run SANA news agency said.
The office of Russia's trade mission in Damascus was damaged Tuesday by a mortar fired from Eastern Ghouta, the Russian Foreign Ministry said, according to SANA.
Israeli missiles claim
Israeli warplanes fired several missiles from Lebanese airspace targeting a Syrian military position in the Damascus countryside early Wednesday, according to a statement from SANA, citing the Syrian regime.
"The army air defense intercepted the missiles and destroyed most of them," SANA said, quoting a Syrian army statement.
There was no mention of any damage caused or casualties.
The Israel Defense Forces (IDF) declined to comment on the Syrian reports.
On Tuesday, Israeli Prime Minister Benjamin Netanyahu and members of the Israeli Security Cabinet visited the Israeli-occupied Golan Heights where they were briefed by senior military officials. Israel captured the Golan Heights from Syria in 1967 and annexed it in 1981.
"I am impressed by the major work the IDF is doing to defend our borders and our state," Netanyahu tweeted. "We want peace but are prepared for any scenario and I suggest that no one test us."
Meanwhile, Turkey's Anadolu news agency reported that Turkish Foreign Minister Mevlut Cavusoglu had arrived Wednesday in Tehran for talks likely to focus on Syria, among other regional issues.
Iran's official news agency IRNA said he would meet with Iranian President Hassan Rouhani and Foreign Minister Mohammad Javad Zarif.
Turkey last month launched a military operation in northern Syria aimed at ending the US-allied Kurdish militia control of Afrin and the surrounding region along the Syrian-Turkish border.