Syria has now received even more firepower from Russia. President Putin has now added drones, attack helicopters and aircraft to the forces in Syria.

The new arsenal includes a dozen Su-24 Fencer and a dozen Su-25 Frogfoot ground-attack planes that were delivered to an air base near Latakia, Syria. The base now holds 28 warplanes according to a senior United States official. Previously, it only had four Flanker air-to-air fighters.

The new equipment is also protected by two or possibly three SA-22 antiaircraft systems and unarmed surveillance drones.

U.S. officials reported that Russia has 15 helicopters, nine tanks, three surface-to-air missile systems, over 25 fighter and attack aircraft and at least 500 ground troops in Syria at this time.

“The equipment and personnel just keep flowing in,” an official told the New York Times. “They were very busy over the weekend.”

U.S. and Russian officials have met to discuss the escalation of Russia’s military power in Syria, but there has been no follow-up contact since last Friday. The U.S. is worried that Russia is more concerned with bolstering the Assad regime rather than eliminating the ISIS threat. The U.S. believes that Syrian President Bashar al-Assad should step down from his leadership role in order to resolve the civil war.

“If Russia looks to play a constructive role against [ISIS], that’s one thing, but if what they’re doing is, in fact, propping up the Assad regime, then that’s an entirely different issue altogether,” stated U.S. State Department spokesman John Kirby. He added: “because it is the Assad regime that has been a magnet for extremists inside Syria.”

While Russia has not flown any combat missions using jets or helicopters in Syria at this time, U.S. officials stated they will be keeping a close eye on all Russian military moves.