Updated

The Syrian military reportedly has started using new air and ground weapons supplied by Russia as part of a boost in support for Syrian leader Bashar Assad that has alarmed U.S. officials.

Reuters, citing an unnamed Syrian military source, reported Thursday the Syrian army had been training to use the weapons in the last few months and was just now starting to deploy them. However, the source didn’t specify what type of new weapons Syria was using.

“The weapons are highly effective and very accurate, and hit targets precisely," the source told Reuters. "We can say they are all types of weapons, be it air or ground."

Syrian Foreign Minister Walid al-Moallem said Thursday that Russia had provided weapons and trained Syrian troops in how to use them. He said the Assad government would be willing to ask Russian troops to fight alongside Syria as it battles the Islamic State group (ISIS).

Russia has repeatedly said that its supplying of weapons and other support for Assad’s regime is aimed at fighting terrorism and preventing a “total catastrophe” in the region. Support also includes a large Russian military presence, but the full scale of the aid still remains unclear.

U.S. officials believe Russian buildup in Syria could only exacerbate ongoing bloodshed in the region. Pentagon spokesman Peter Cook condemned Russian assistance in Syria, saying it would be “counterproductive to the ultimate solution in Syria, which we think is a political and diplomatic solution, not a military solution."

Meanwhile, Syrian warplanes carried out a rare wave of airstrikes in ISIS’ self-declared capital of Raqqa, activists said Thursday. The attacks coincided with a visit by a U.N. envoy to Damascus.

Syrian helicopters dropped barrel bombs, striking a marketplace killing 17 people. An activist in the rebel-held Bosra Sham said the explosives were dropped as the market was packed with shoppers and people buying school supplies for children due to return to the classroom this week.

The activist claims about 24 people were killed and several others wounded. The Britain-based Syrian Observatory for Human Rights put the death toll at 17. Such discrepancies are common in the aftermath of large bombings.

Raqqa and other cities in the region are often targeted by the U.S.-led coalition fighting ISIS militants. The Syrians have occasionally targeted the city as well, but usually Syrian airstrikes target rebel-held areas of the country.

Assad reiterated in an interview with Russian media Wednesday that the U.S. refuses to coordinate with his government to fight ISIS.

Assad claimed that if U.S. officials, "cooperate with the Syrian army, this is like recognition of our effectiveness in fighting ISIS. He also said that his priority is "defeating terrorism" in Syria.

The U.S. has agreed to meet with Russia amid the belief that Russia is sending military equipment to prop up Assad, rather than help him fight ISIS. U.S. Secretary of State John Kerry revealed the offer game Wednesday during a phone call with Russian Foreign Minister Sergey Lavrov. Kerry said he personally thought the idea was a good one, but stressed that the administration was still weighing its merits. Other officials said later that national security adviser Susan Rice had signed off on the talks and that Defense Secretary Ash Carter had agreed.

Thursday's airstrikes on Raqqa coincided with a visit by the U.N. envoy to Syria, Staffan de Mistura, to Damascus. De Mistura met with Foreign Minister Walid al-Moallem, who repeated to him that fighting terrorism was the Syrian government's priority, according to the state-run news agency SANA.

More than 250,000 people have been killed in Syria's civil war, according to U.N. officials.

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The Associated Press contributed to this report.