Ukraine issued a warning on Saturday about a new threat from the Russian military as it seeks to launch a new counteroffensive to reclaim key areas in the coming months.

The warning comes more than a year after Russian President Vladimir Putin launched his “special military operation” on Ukraine last February. Kremlin military leaders initially hoped for a quick victory over Ukraine, which at the time was perceived to have a smaller, weaker military. However, Ukraine’s stronger-than-expected defense effort, bolstered by weaponry provided by key allies including the United States, has blunted Russian military gains.

Russia’s invasion has stalled throughout the winter, and Ukrainian military leaders have in recent weeks indicated they may move toward launching a new counteroffensive, though little detailed information, including which areas would be specifically targeted, remain publicly unknown.

Ahead of any new military action, Ukrainian military leaders are now warning that Russian forces are using modified FAB-500 aerial bombs against areas near the front lines.

Yurii Ihnat, spokesman for the Air Force of the Armed Forces of Ukraine, in a recent newscast warned that the Russian military has been converting FAB-500 high-explosive aerial bombs into cruise missiles. They are using these bombs “from a distance that is unreachable for Ukrainian air defense,” according to a report from Ukrainska Pravda, a Ukrainian online newspaper.

Ihnat added that Ukraine believes Russia may be preparing to ramp up production of these bombs, which will be equipped with wings and GPS navigation, according to the news outlet.

Mykola Oleshchuk, the commander of the Air Force of the Armed Forces of Ukraine, also warned about the use of these bombs.

“Increasingly, the enemy is using guided air bombs weighing 500 kg along the entire front line. There are signs of preparation for the mass use of 1,500 kg KABs,” Oleshchuk warned in a Saturday Telegram post. “The planes of the Russians do not enter the zone of damage of our air defense, striking remotely at the front line and near-frontline cities. Peaceful residents in the northern, southern, and eastern regions are suffering.”

Oleshchuk continued to write that F-16 fighter jets would enable Ukraine to more effectively combat these bombs.

Ukraine has urged its Western allies, including the U.S., to provide it with more powerful fighter jets to boost its defense efforts. The war-torn country has specifically pushed for F-16 “Fighting Falcon” jets. However, the Biden administration has thus far declined to provide them to Ukraine.

“The F-16 is armed with air-to-air missiles with a range of up to 180 km. This will make it possible to drive enemy aircraft away from our borders and the line of combat, which will minimize the likelihood of using guided aerial bombs and other air-based weapons,” Oleshchuk wrote.