As Russian-backed separatists in Ukraine dig in for a long war, they’re relying more and more on heavy artillery to match Kiev’s army.

Howitzers are nothing new to the conflict, and the drumbeat of artillery explosions has become part of eastern Ukraine’s ambient soundscape.

But within the past few days, the rebels have received a lot more of the big guns—and most likely from Russia.

Videos uploaded to social media showed one convoy near the town of Snizhne—near the Russian border—consisting of nearly 40 unmarked Kamaz military trucks hauling 18 towed artillery pieces. Owing to their size, ballistic shields and wheels, the guns resemble 2A65 Msta-B howitzers.

These 152-millimeter guns are some of the biggest in the Russian inventory, and have more than double the range of the separatists’ 122-millimeter D-30 guns.

The Associated Press spotted a similar convoy in Snizhne.

Several of the vehicles resemble support and radio trucks, because of their conspicuous shielded exteriors used to block electromagnetic interference.

The convoy included a single unarmed BTR-80—likely a command or forward observer vehicle for the artillery. This makes the convoy roughly the size of a Russian artillery battalion.

On the same day, monitors from Organization for Security and Cooperation in Europe witnessed the same convoy in Makeevka—on the road from Snizhne—headed west toward Donetsk. But the OSCE described the convoy as having smaller 122-millimeter guns, an OSCE spot report noted.

Either way, that’s a lot of guns, and just one of three convoys on the move in Ukraine on Nov. 8. The others included tanks—in a sign of even more heavy weapons pouring into the conflict.

The guns might give the separatists enough firepower to dislodge Ukrainian troops holding the strategic Donetsk Sergey Prokofiev International Airport just outside the city. If the rebels are trying to establish a breakaway state based in Donetsk, they need that airport.

Sunday morning marked some of the heaviest fighting in Ukraine in a month, with the worst centered around the airport, according to the Wall Street Journal. Ukrainian activists are also fearing a major offensive.

In his daily updates to Facebook, pro-Ukrainian activist Dmitry Tymchuk described separatist and Russian troops—regular army, airborne and military intelligence personnel—reorganizing into tactical assault groups.

There have been more probes of Ukrainian defenses, and Russian artillery crews appear to be refining their targeting skills.