A Syrian rebel and a war monitor said Russian air strikes had resumed against insurgents in Syria's northwestern Idlib province on Tuesday after several weeks.
U.S. President Donald Trump late on Monday warned Syrian President Bashar al-Assad and his allies Iran and Russia not to "recklessly attack" Syria's rebel-held Idlib province, warning that hundreds of thousands of people could be killed.
"The Russians and Iranians would be making a grave humanitarian mistake to take part in this potential human tragedy. Hundreds of thousands of people could be killed. Don’t let that happen!" Trump wrote in a tweet.
A source has told Reuters that Assad is preparing a phased offensive to regain Idlib.
The Kremlin dismissed Trump's warning to Syria, saying on Tuesday the area was a "nest of terrorists".
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Trump on Monday warned Syrian President Bashar al-Assad and his allies Iran and Russia not to "recklessly attack" the area, saying hundreds of thousands of people could be killed.
"Just to speak out with some warnings, without taking into account the very dangerous, negative potential for the whole situation in Syria, is probably not a full, comprehensive approach," Kremlin spokesman Dmitry Peskov said.
Fearing a chemical attack
Russia, last week, again accused Syrian rebels of preparing a chemical attack which Moscow says will be used to justify a Western strike against Syrian troops.
Deputy Foreign Minister Sergei Ryabkov said last Tuesday that the al-Qaida-linked Levant Liberation Committee is preparing the attack in the northern Idlib province, and that it would be filmed by first-responders known as the White Helmets, who Russia has accused of fabricating past attacks.
Also last Tuesday, senior U.S. officials warned the Russian and Syrian governments against chemical weapons use in Syria as forces allied with its President Bashar Assad prepare for an offensive on a rebel stronghold.
The battle for Idlib
The northern province and surrounding areas are the last major enclave held by insurgents fighting Assad, who has been backed by both Russian and Iranian forces in Syria's seven-year-old civil war. They are home to some three million civilians.
Trump has sought better relations with Russia since taking office in 2017 but the United States has been unable to rein in Moscow's military and diplomatic support for Assad.
Assad has sworn to recapture every inch of Syria and has made big gains against rebels since Russia joined his war effort in 2015.
Last week, a source close to Damascus said the government was preparing a phased offensive to recover Idlib province, but Turkey, whose army has a string of observation posts around the edge of the rebel area, has warned against such an assault.
Russian air raids ceased in and around Idlib on August 15, but pro-Syrian government forces have maintained an aerial bombardment and shelling against rebels there, the war monitor, the Syrian Observatory for Human Rights, said.
The rebel source and the Observatory both said the air strikes were in the countryside near Jisr al-Shughour on the western edge of the rebels' northwestern territory.
Russian, Turkish and Iranian leaders are due to meet on Sept. 7 in Iran and are expected to discuss the situation in northwestern Syria.
The Associated Press contributed to this report