The Russian parliament on Friday debated ratifying a treaty with Syria that allows Russian troops to stay indefinitely in the Mideast country, with lawmakers strongly backing the deal in a show of support for embattled Syrian President Bashar Assad.

The development comes against the backdrop of a Syrian army onslaught, backed by Russian warplanes, on the rebel-held eastern part of the city of Aleppo in northern Syria. Since the collapse of a U.S.-Russia-brokered cease-fire last month, ties between Moscow and Washington have grown even more strained.

The two powers support opposite sides in the war — Moscow has been a staunch Assad ally and the United States backs Syrian rebels trying to oust him.

Russia's air campaign in Syria, launched a year ago, has reversed the tide of war and helped Assad's forces regain some key ground. Moscow says the goal of its military operation is to assist the Syrian army in the fight against terrorism and rejects accusations of targeting civilians in relentless airstrikes.

Lawmakers in the Kremlin-controlled State Duma are to vote on the deal later in the day.

The treaty allows Russia to keep its forces at the Hemeimeem air base in Syria's coastal province of Latakia, Assad's Alawite heartland, as long as it wants. The deal was signed in August 2015 in Damascus, a month before the Russian air campaign began.

Russia also has a naval base in Syria's port of Tartus, the only such outpost outside the former Soviet Union. That base is not covered by the treaty, and some lawmakers said it could be subject of a separate deal.

On Thursday, the Russian military warned the U.S. against striking the Syrian army, underlining that its air defense weapons in Syria stand ready to fend off any attack.

Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said any U.S. strikes on areas controlled by Assad's government could jeopardize the lives of Russian servicemen and noted that the Russian S-300 and S-400 air defense missile systems have a range that would be a "surprise" to any country operating its aircraft over Syria.

On Friday, Russian lawmakers suggested Russia should demonstrate its military muscles to deter the U.S. Leonid Kalashnikov, a Communist lawmaker, said Russia must "seriously defend our interests in Syria."

Meanwhile, Syrian rebels said they had captured several Shiite pro-government fighters in Aleppo after repelling a government offensive on a southern neighborhood of the contested city.

The Fastiqum rebel faction and social media postings showed rebels harshly interrogating three captives, who spoke with Iraqi accents.

Fastiqum and Nour el-Dine el-Zinki groups said the prisoners were fighters for the Iraqi Nujaba Shiite militia, which sent some 4,000 fighters recently to Syria to fight alongside government forces in Aleppo, a militia official told the AP earlier this week. He spoke on condition of anonymity because he was not authorized to discuss the militia's strategy.

The Britain-based Syrian Observatory for Human Rights, which monitors the conflict through activists on the ground, reported that five presumed Iraqi militiamen had been taken prisoner by the rebels.

Syria's government depends on an array of foreign militias, elite Iranian fighters, as well as Russian military intelligence and air power to maintain its fight against rebels, who also rely on foreign fighters.

The Observatory said clashes were underway Friday in Aleppo's southern Sheikh Saeed neighborhood, as well as along other fronts inside the city.