At least 53 civilians, including 21 children, perished early Sunday morning when Russian air strikes hit "residential buildings" in a village held by the Islamic State group in eastern Syria, a monitor said.
The Britain-based Syrian Observatory for Human Rights said the strikes hit the village of Al-Shafah in Deir Ezzor province, on the eastern bank of the Euphrates River.
The monitor had initially given a death toll of 34 civilians but the number spiked after more bodies were recovered.
"The toll increased after removing the debris in a long day of rescue operation," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights told AFP, adding the strikes hit "residential buildings."
At least 18 people were also wounded in the air raids, he added.
The Observatory relies on a network of sources inside Syria, and says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.
Russia is a close ally of Syria's President Bashar al-Assad, and in September 2015 began a military intervention in support of his government that has gradually helped Damascus regain territory.
Syria's Deir Ezzor is one of the last places Islamic State jihadists hold territory in the country, after being driven from their major strongholds including their one-time de facto Syrian capital Raqa city.
The oil-rich eastern province that borders Iraq was once almost completely under Islamic State control, but the jihadists now hold just nine percent of Deir Ezzor, according to the Observatory.
They have faced two separate offensives there, one led by the regime with Russian backing and the other by the US-backed Syrian Democratic Forces, an alliance of Kurdish and Arab rebel fighters.
However, recent reports claim that the US has agreed to Turkey's request to stop supporting the Kurdish Democratic Forces (SDF). Despite this, a senior official among the rebels refuted this, and told the Al-Quds Al-Arabi newspaper that they consider the US a true partner that would not go back on its promise.
More than 340,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.
Netanyahu 'insists on Israel's security' with Putin
As part of a round of talks that Russian President Vladimir Putin conducted with world leaders last week, Prime Minister Benjamin Netanyahu spoke with him on the phone on Tuesday for half an hour. The Prime Minister's Office said that the phone conversation dealt with Syria and Iran's attempts to establish itself in Syrian territory, with the PMO stating that "Prime Minister Netanyahu insisted on Israel's security principles."
Earlier on Tuesday, the White House announced that US President Donald Trump and his Russian counterpart talked for an hour on the phone, discussing events in Syria, Ukraine, Iran, North Korea and Afghanistan. Putin also spoke with Saudi King Salman bin Abdulaziz Al Saud.
Russia in the driving seatThe latest strikes come as the United Nations tries to revitalise its flagging efforts to end a six-year civil war that has left Syria devastated and huge swathes of its population refugees.
On Tuesday, the eighth round of UN-brokered talks will kick off.
They have achieved little so far, but may be bolstered by the opposition's decision to bring a unified delegation to Geneva for the first time.
For progress to happen rival sides will need to overcome the hurdle that has derailed past discussions: the fate of Syrian President Assad.
Within Syria, however, Assad is loathed by much of the rebel opposition, who want him gone.
Backed by Russia's decisive military support, Assad's government has regained control of 55 percent of the country, including major cities Damascus, Aleppo, Homs and Hama, and around two-thirds of the population lives in regime-held areas.
The rest is carved up between rebel factions, jihadists and Kurdish forces.
Some experts believe that Russia has clearly put itself in the driver's seat in recent months, especially as US President Donald Trump's administration has pulled back from Syrian diplomatic front.
An axis of non-western power
Russia, fellow regime ally Iran and rebel-backer Turkey have hosted negotiations in the Kazakh capital Astana that led to the creation of four "de-escalation zones" which produced a drop in violence, though deadly air strikes and battles continue in some areas.
Last week, Putin called for a "congress" of Syrian regime and opposition figures, a move backed by Ankara and Tehran during Putin's summit meeting with Iranian President Hassan Rouhani and Turkish President Recep Tayyip Erdoğan.
Putin told Ruohani and Erdoğan that ISIS's occupation of Syria had been thwarted by Iran and Turkey, noting that thanks to their three countries, "Syria's collapse was prevented."
Following the Putin-Ruohani-Erdoğan summit leaders' meeting, Putin announced that 98 percent of Syrian territory has returned to Assad's control.
At the end of the month, all in the Syrian conflict are set to meet in Geneva to begin a series of talks that will determine Syria's future: these include consolidating its territory, rebuilding its ruins after six years of civil war and policing its affairs in the near future.