Two Russian Sukhoi-34 planes on Tuesday wiped out a workshop of Jabhat al-Nusra (outlawed in Russia) in Syria's Idlib province where the militants were making attack drones and kept explosives in store.
Turkey and the U.S. said they were racing find a diplomatic solution to avoid "a serious massacre" in Syria's Idlib province ahead of a critical summit on Friday which is likely to decide the fate of the rebel stronghold.
His comments came as Syrian troops gathered near the north-western region of Idlib for a major assault that has raised fears of a humanitarian catastrophe on a scale not yet seen in the seven-year-old conflict.
Russian Federation has been a key supporter of Assad and complained about militants in Idlib targeting its own facilities.
De Mistura urged Russian President Vladimir Putin and his Turkish counterpart Recep Tayyip Erdogan to urgently speak on the phone even before they are set to meet with their Iranian counterpart in Tehran on Friday. Hundreds of thousands of people could be killed.
Imran Khan set to change Pakistan - Editorial One
He stressed that he wanted to improve relations with the neighbouring countries, including India. However, experts believe that Pakistan might be unable to support Iran as it has promised.
Tuesday's bombardment hit several areas held by the jihadist-led Hayat Tahrir al-Sham alliance, among them the large town of Jisr al-Shughur, but also areas held by rival Turkish-backed rebels, including the town of Ariha.
De Mistura said there were about 10,000 fighters with UN-recognized terrorist organizations now in Idlib.
The Syrian military has been sending reinforcements to the area for more than a month, and Russian Federation has ramped up its rhetoric. But he stressed that there are some 2.9 million civilians in the province, including around one million children, who "are not terrorists".
FILE - A Syrian girl holds an oxygen mask over the face of an infant at a make-shift hospital following a reported gas attack on the rebel-held besieged town of Douma in the eastern Ghouta region on the outskirts of the capital Damascus on January 22, 2018.
The Trump administration said Tuesday that any reckless action taken by the President Bashar al-Assad and his regime's allies would incur the United States' ire.
The president called Defense Secretary Jim Mattis to issue the deadly order. On the other hand, Mattis said there is evidence that President Bashar al-Assad has "done this before and we are watching very closely for this". "We're going to be much more measured", Woodward wrote.