BEIRUT, Lebanon (CNN) -- More than 1,000 children have been killed or injured this year across Syria, a United Nations Children's Fund official told CNN on Tuesday.
The grim statistic, from UNICEF regional communications chief Juliette Touma, comes as violence rips through the rebel-held enclave of Eastern Ghouta, where about a third of the nearly 400,000 besieged civilians are children, according to the agency.
In all, 342 children were killed and 803 were injured in Syria in the first two months of 2018, Touma said, citing multiple sources.
About 100 people were killed Monday in Eastern Ghouta, a suburb of Damascus, making it the deadliest day there since the United Nations Security Council passed a nationwide ceasefire resolution on February 24, the Syrian American Medical Society said Tuesday in a statement.
Almost 600 people are believed to have been killed and more than 2,000 injured since Syrian government forces launched an air and ground offensive on Eastern Ghouta on February 18, according to the United Nations.
Sitting in underground shelters has become the "new norm," UNICEF officials said, and activists' footage of basements show the shelters to be ill-equipped and filthy.
Rebels refuse Russia's offer
Russia has offered rebels and their families safe passage out of Eastern Ghouta, promising to protect insurgents from prosecution, Russia's Defense Ministry said Tuesday in a Facebook post. The Russian military would also provide transport and security, the statement said.
One of two main rebel groups in Eastern Ghouta swiftly refused Russia's offer.
"There is no contact with the Russians at all, and no official offer was made. This issue is being circulated by the media. The rebels in Eastern Ghouta, including Faylaq al-Rahman, categorically refuse to leave Ghouta," the armed group's spokesman, Waiel Olwan, told CNN.
"This is against Security Council Resolution 2401, that was unanimously adopted on 24 February 2018, that dictates the safe passage of aid into Eastern Ghouta and a 30-day truce," he added.
Russia -- the Syrian government's most powerful ally -- has accused rebels of preventing civilians from leaving Eastern Ghouta. Last week, Russian President Vladimir Putin ordered a daily humanitarian pause in the fighting in the Damascus suburb and the opening of a humanitarian corridor.
Only a handful of civilians have reportedly been evacuated. The Syrian government and the rebels have traded blame over the corridor's closure, each arguing that shelling from the other side has prevented the safe passage of civilians.
On Tuesday, 39 people were killed when a Russian transport plane crashed while trying to land at Hmeimin airbase near Latakia, Syria, about 150 miles (241 kilometers) north of Damascus, according to Russian state media, Tass, which cited the Russian Defense Ministry.
Immediate reports in the aftermath of the crash had put the death toll at 32.
Aid convoy forced to leave early
On Monday, a convoy carrying much-needed food and medical supplies entered the rebel enclave for the first time since the Syrian government launched a deadly offensive on the area two weeks ago.
The 46-truck aid convoy delivered the aid amid shelling but had to pull out before unloading everything, said Sajjad Malik, the United Nations refugee agency representative in Syria.
"We delivered as much as we could amidst shelling. Civilians are caught in a tragic situation," Malik said in a tweet.
The Syrian government initially barred most medical supplies in the aid convoy from getting through, according to Linda Tom, spokeswoman for the United Nations Office for the Coordination of Humanitarian Affairs in Syria.
On Sunday, government forces took control of several villages inside Eastern Ghouta, marking a major turning point in the offensive.
Eastern Ghouta is one of the last major rebel-held enclaves in Syria, which has been ravaged by war for almost seven years.
CNN's Tamara Qiblawi and Ghazi Balkiz reported from Beirut and Sebastian Shukla reported from Moscow. CNN's Sarah El Sirgany contributed to this report from Abu Dhabi.
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