The U.N. has warned that an offensive could "create a humanitarian emergency at a scale not yet seen through this crisis."
Syrian and Russian fighter jets carried out more than 20 airstrikes in towns near Idlib on Tuesday, ahead of an expected government offensive designed to drive out the rebels who control the Syrian city.
It was the first time in more than three weeks that Russia was involved in airstrikes in the Idlib area, according to the the Syrian Observatory for Human Rights, a U.K.-based monitoring group.
Four people were killed in strikes in and around Jisr al-Shughur, according to activist Hadi Khrat. The town is located 30 miles southwest of Idlib.
Idlib is the country's last remaining rebel-held enclave. Nearly 3 million civilians live thre — including many refugees from other war-torn parts of Syria. However, thousands of its residents are believed to belong to powerful extremist groups, prompting fears that civilians will be caught up in the violence.
A Russian-backed government offensive has been predicted for weeks as Syrian President Bashar al-Assad continues with his quest to regain control of the country.
Rebels have spent recent days training for the expected offensive.
Russia's Tass news agency on Tuesday quoted Vladimir Putin's spokesman Dmitry Peskov as saying that militants in Idlib "must be sorted out."
The airstrikes came hours after President Donald Trump warned Assad not to "recklessly attack" the area.
"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 tweeted late Monday.
Trump's message came after Secretary of State Mike Pompeo said on Friday that the U.S. would view an attack on Idlib as "an escalation of an already dangerous conflict."
The U.S. has made clear its concern that Assad could use chemical weapons, with National Security Adviser John Bolton saying that the U.S. "will respond very strongly."
Assad has publicly called for reconciliation and vowed to target only "terrorists" in Idlib. However, the United Nations Children's Fund (UNICEF) estimates more than a million children are at risk in the event of a government assault.
The fighting has the potential to "create a humanitarian emergency at a scale not yet seen through this crisis," John Ging, the director of operations for the U.N. humanitarian affairs office, warned last week.
Fearing a humanitarian crisis on its border, Turkey says it is seeking to ward off a full-scale offensive on Idlib and has pressed Russia and Iran to allow more time to separate "radical militants" from the armed groups it has backed.
"We are working toward stopping this attack," Turkish Foreign Minister Mevlut Cavusoglu said in Vienna on Friday.
Turkey's defense minister and intelligence chief have visited Moscow while Iran's foreign minister has held meetings in Ankara over the past few days, according to the Associated Press.
Iranian, Russian and Turkish leaders are expected to meet up in Iran next week, while their representatives meet in Geneva on Sept. 10 and 11.
In the past six months, around 500,000 people have reportedly arrived in Idlib after fleeing fighting in eastern Ghouta and other opposition-held areas, according to the U.N.
During the seven-year revolt, millions of people have been driven from their homes across Syria and around 400,000 are believed to have been killed.