Syrian government forces, with support from Russia, are planning an operation to retake Aleppo and battle an Islamist rebel offensive in the region, the Syrian prime minister said Sunday after a week of clashes.

The use of Russian firepower in the Syrian government offensive to retake Syria’s largest city, which has been partly controlled by rebels since 2012, clouds the future of upcoming peace talks.

“We, together with our Russian partners, are readying ourselves for an operation to liberate Aleppo and blockade all illegal armed groups which have not signed up to or are violating a cease-fire,” said Syrian Prime Minister Wael al Halqi in a meeting with Russian parliamentarians, Russian news agency Tass reported.

The regime and rebel groups agreed to a cease-fire that came into effect in late February. But it doesn’t cover Islamic State and al-Qaeda-linked Nusra Front, which joined with other Syrian rebels to launch an offensive last week to recapture territory near Aleppo that had previously been retaken by forces loyal to Syrian President Bashar al-Assad. On Tuesday, Islamist rebels shot down a government warplane and captured its pilot.

Sunday’s announcement comes a month after Russia said it was pulling back a large part of its air forces from Syria, when President Vladimir Putin said the Kremlin’s mission in Syria was largely complete.

Russian firepower is widely seen as having reversed the fortunes of Mr. Assad on the battlefield, winning back the Damascus regime swaths of land from moderate and Islamist rebels. Russia said it would maintain a contingent of jet fighters in the country to keep up Moscow’s fight against Islamist groups.

Clashes between rebels and pro-regime forces in the southern Aleppo countryside have been ongoing for a week.

Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said Nusra Front militants had accumulated forces around Aleppo and had increased their military operations against government troops and civilian quarters around the city.

Local antigovernment activists said rebels who had briefly seized new ground were forced to withdraw Sunday morning due to intensified Russian airstrikes.

The fighting killed at least 19 rebel fighters and 16 regime soldiers, according to the Syrian Observatory for Human Rights, a U.K.-based opposition monitoring group.

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