Smoke billows following a reported Russian air strike in the northwestern Syrian province of Idlib, on October 24. Photo: abdulaziz ketaz/Agence France-Presse/Getty Images

Russian aircraft carried out strikes on the last rebel stronghold in northwestern Syria early Thursday, according to a monitoring group and rescue workers, raising fears of an all-out Syrian regime offensive to retake the area following a Moscow-brokered border deal with Turkey.

The airstrikes targeted the Idlib, Hama and Latakia provinces amid intense artillery shelling and rocket fire, according to the independent Syrian Observatory for Human Rights. There were no immediate reports of casualties.

The Russian strikes raised fears that Idlib was part of a broader bargain agreement between Moscow and Ankara to delineate their influence in Syria. Terms of the main pact were announced after Tuesday’s summit between Russian President Vladimir Putin and his Turkish counterpart Recep Tayyip Erdogan and covered a large Kurdish-controlled area in Syria’s northeast. While Idlib wasn’t part of the agreement, Turkish officials have said they feared that securing Russia’s support to establish a safe zone in the northeast could invite a resumed offensive by Russian aircraft and Syrian regime troops on Idlib.

Russian troops are also filling a vacuum left by U.S. soldiers in northeastern Syria after President Trump ordered them to exit Syria.

Russian planes carried out 32 strikes throughout the day, the Syrian Observatory for Human Rights said.

The Russian Ministry of Defense didn’t immediately respond to a request for comment. Moscow says that “illegal armed units” are continuing to violate a truce in Idlib province.

The opposition-held region is under a demilitarization agreement between Russia and Turkey since last year that has provided it a degree of protection from the Syrian regime. Syrian President Bashar al-Assad, backed by Russia and Iran, has vowed to retake Idlib and its surrounding areas as he attempts to reassert control over the entire country after more than eight years of conflict.

The regime and Russia have regularly launched strikes in the opposition-held area, which is home to more than three million people, but refrained so far from launching an all-out ground offensive that the United Nations has long warned would cause a humanitarian disaster.

Turkey, which is already host to an estimated four million refugees from Syria, fears a battle for Idlib would send even more refugees toward its border.

Turkey and Russia agreed Tuesday to drive out Kurdish militias that Ankara considers terrorists from a “safe zone” in northeastern Syria. Turkey intends to relocate half of the Syrian refugees living in the country to this area.

But now Idlib residents fear that Turkey will look the other way, allowing Russia and the regime to retake the area in an apparent exchange for helping drive Kurdish forces out of the border region in the northeast.

“Ten days ago the airstrikes on Idlib intensified,” said Ahmad Shaykho, a spokesperson for the Syria Civil Defense, a rescue group, in Idlib, noting that the shelling hadn’t stopped since a brief pause in August.

Nine civilians have been killed and 22 have been injured over the past 10 days as a result of Russian and Syrian government strikes, which included barrel bombs dropped by regime aircraft, on Idlib according to rescue workers.

Mr. Erdogan has sounded the alarm about the humanitarian chaos that would result from an all-out offensive on Idlib. He warned of the destabilizing effects of another refugee influx into Turkey.

But in recent weeks, Mr. Erdogan has refrained from commenting on the uptick in air and artillery strikes on the region.

Mr. Assad vowed to retake Idlib during a rare visit to soldiers on the front lines in the province on Tuesday.

“The conclusion of the battle in Idlib is the basis for ending chaos and terrorism across Syria,” he said.

Russia’s deal with Turkey this week has allowed Moscow to consolidate its role as the main power broker in Syria.

Under the pact, Russian and Syrian security forces would oversee the withdrawal of Kurdish fighters from a 300-mile-long strip of land along the Turkish border by Tuesday. After their departure, Turkey and Russia plan to conduct joint patrols in parts of the area.

On Thursday, Russian Deputy Foreign Minister Sergei Vershinin told reporters in Moscow that “Kurdish militias are leaving areas on the Syrian-Turkish border,” according to the Russian news agency Interfax.

Separately, Kurdish officials accused Turkey and allied Syrian forces of attacking three villages despite a pause announced by Turkey on Tuesday.

“Despite our commitment to the cease-fire decision and the withdrawal of our forces from all of the cease-fire areas, the Turkish state and the terrorist factions that are loyal to it continue to violate the cease-fire process and continues with its extermination war against our people and villages,” said Kino Gabriel, a spokesman for the Kurdish-led Syrian Democratic Forces.

Turkey didn’t immediately comment on the claims.

Russian military police started their first patrol in the Syrian town of Kobani near the border with Turkey, Igor Seritsky, a spokesman for the command of Russian forces in Syria, told reporters on Thursday, according to Interfax.

Also speaking to reporters Thursday, Kremlin spokesman Dmitry Peskov said that Moscow doesn’t accept the fact that a limited number of U.S. troops would remain in the region to protect Syria’s oil fields.

Moscow is also demanding that Washington leave the area of the al-Tanf military base in southern Syria, Mr. Vershinin told reporters at a press conference Thursday, as quoted by the official Russian news agency RIA Novosti.

The deputy foreign minister said that the U.S. military was “in this area illegally” and is blocking the road connecting Iraq and Syria. Mr. Vershinin called the presence of the U.S. troops “occupation.”

The al-Tanf base is located on the Syrian-Iraqi border and not far from Jordan, surrounded by Syrian pro-government forces and Iran-backed militias. The base lies on the strategic Damascus-Baghdad highway and acts as a buffer preventing Tehran from connecting the two cities.

The U.S. withdrawal and the Russian-Turkish agreement paved the way for Syrian regime forces to return to the Kurdish-ruled northeast, but the endgame of the conflict still looms in the northwest.

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There, opposition groups are making their last stand against the Assad regime more than eight years after the revolution that spawned the Syrian conflict. The rebels’ territory has collapsed under years of assault by the regime and by Russia, which intervened on the side of the regime in 2015.

When Rifaat Rashid fled his home in Hama’s western countryside, he made sure to settle at the Turkish border.

He said he knew that government troops would one day resume their offensive on Idlib and attempt to capture the entire province. He wanted to make sure he was on the border with Turkey when that happens.

“There are huge fears,” he said. “But as normal people there is nothing we can do.”

Write to Jared Malsin at jared.malsin@wsj.com

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