Russia's air force appears to be backing off after Ukraine said it shot down 15 planes in 2 weeks: ISW

• Russia is reducing its military flights in the Ukraine war, a report said. 
• It comes Ukraine shot down a series of Russian planes.
• Ukraine has used a range of tactics to counter Russia's air dominance. 

A Ukrainian kill streak against Russian fighter jets appears to have caused Russia's air force to scale back on flights, a think tank said.

Ukraine managed to take down 15 of Russia's combat jets in two weeks, according to Ukrainian officials.

The Institute for the Study of War, a US think tank, said Sunday that "relatively high Russian aviation losses appear to be prompting a significant decrease in Russian aviation activity in eastern Ukraine."

Ukrainian Air Force Commander Lieutenant General Mykola Oleshchuk on Saturday said that all Russian flights had been halted in the region after 7 p.m., after the downing of Russian Su-34 aircraft.

Ukrainian Air Force Spokesperson Colonel Yuriy Ihnat said the decrease in flights continued on Sunday.

Previous Russian combat jet losses have caused its air force to temporarily reduce aviation activity in Ukraine.

In a December update, for instance, the ISW said that Russia was pulling back on aviation activity after Ukraine said it took out three of its jets.

The ISW noted that it's unclear how long the latest reduction in Russian military aviation activity will last.

"Russian forces appeared to tolerate an increased rate of aviation losses in recent weeks in order to conduct glide bomb strikes in support of ongoing Russian offensive operations in eastern Ukraine, and the Russian command may decide in the future to assume the risk of continued aviation losses in pursuit of further tactical gains," it added.

Glide bombs have flight control systems that allow attacking aircraft to release them at a distance from the target.

Analysts at the International Institute for Strategic Studies said this month that Russian aircraft appear to be carrying glide bombs more "to provide a greater stand-off distance to avoid further losses."

But as Business Insider's Sinead Baker reported last week, this strategy may explain why Ukraine seems able to shoot down more jets.

Getting unpowered glide bombs to travel far enough from a distance requires "lobbing from very, very high altitudes and speeds," Justin Bronk, a Russia and air warfare expert at the UK's Royal United Services Institute told BI.

This means that Russian jets must fly higher, allowing Ukraine more time to detect and target them, he added.

Ukraine has used a range of tactics during the war to prevent the significantly larger Russian air force from achieving total control of the skies, including air defense systems provided by its Western allies and the effective use of its own fighter jet fleet.

But on the ground, Ukraine's forces are facing serious ammunition shortages, as Republicans in Congress block a $60 billion aid. Russia is seeking to take advantage of the situation by intensifying its attacks.