Air strikes on a village in Syria's rebel-held Idlib province have killed at least 44 people.
It is the highest death toll in a single attack on the region this year, the Syrian Observatory for Human Rights has said.
The monitoring group said Russian war planes probably carried out the attacks.
The Russian Defense Ministry denied its war planes were involved, according to Russian news agencies.
Russia is Syrian President Bashar al-Assad's main ally in his war against an armed opposition, now in its seventh year.
The British-based observatory said the jets targeted the village of Zardana in northern rural Idlib province overnight, killing 27 men, 11 women and six children.
The death toll is expected to increase, since some of the 60 injured in the strikes are in a critical condition, the observatory said.
Rescue workers are searching the rubble for survivors.
US defence secretary says quitting Syria too soon would be a 'blunder'
US Defence Secretary James Mattis has warned it would be a "strategic blunder" to pull out of Syria before United Nations-led peace efforts have made progress.
A US-led coalition is conducting military operations against the so-called Islamic State group in Syria.
Mr Mattis said they must not leave a "vacuum" that President Assad and his Russian and Iranian allies could take advantage of.
Talks in Geneva led by UN Special Envoy Staffan de Mistura have made little headway, but Mr Mattis said they must be given the chance to succeed.
IS seized parts of a town on the Syria-Iraq border today in the latest in a string of attacks.
Germany issues international arrest warrant for top Assad officer - report
It is reported that German prosecutors have issued an international arrest warrant for a top Syrian intelligence officer accused of overseeing the torture and murder of hundreds of detainees.
German news weekly Der Spiegel said that Jamil Hassan, the head of Syria's airforce intelligence directorate and a close ally of Syrian President Bashar al-Assad, is wanted on charges of crimes against humanity.
The magazine described the warrant for Mr Hassan's arrest as "the most serious effort yet by a Western country" to hold the Assad regime accountable for crimes committed against Syrians since a 2011 uprising sparked a brutal civil war.
Clemence Bectarte, lawyer and coordinator for the FIDH Worldwide Movement for Human Rights, said it was "the first time in the world that an independent judiciary has determined individual responsibility for the large-scale crimes perpetrated" by Damascus.
The German federal prosecutor's office declined to comment.
Der Spiegel said prosecutors are alleging that 64-year-old Mr Hassan authorised his officers to "beat, rape, torture and kill" hundreds of prisoners in Syrian government detention facilities between 2011 and 2013.
The charges against Mr Hassan are reportedly based in part on witness reports and photographic evidence smuggled out of the country by a photographer who worked for the Syrian military police.
Known as "Caesar", the photographer fled his homeland in 2013 carrying 55,000 photographs showing the bodies of people who had been tortured.
Although the alleged abuses did not happen in Germany, the case has been filed under the legal principle of universal jurisdiction, meaning that any country can pursue perpetrators regardless of where the crime was committed.
Germany is one of the few countries in the world to apply the universal jurisdiction principle.
The European Centre for Constitutional and Human Rights, a Berlin-based rights group, said the warrant for Mr Hassan's arrest came after it filed a criminal complaint last year against ten high-ranking Syrian officials accusing them of crimes against humanity and war crimes.
Amnesty International also welcomed the arrest warrant.