On Thursday, unverified video footage posted on social media by Syrian political activists showed a pair of new Su-57s being escorted by more conventional Su-35 fighter jets and four Su-25 strike aircraft.

The single-seat, twin-engine planes were allegedly filmed nearby Russia’s Khmeimim Air Base near the southeastern city of Latakia along the Mediterranean coastline and employ state-of-the-art stealth technology to avoid radar detection. They were developed to challenge the U.S. F-22 Raptor — and have been called the “F-22 killer.”

While the new aircraft made its maiden flight last December, military analysts reacted with surprise to Thursday’s sighting, which neither Russian or Syrian officials have officially confirmed.

News sites supporting Syrian President Bashar Assad, however, were quick to welcome the additional firepower to the seven-year-old conflict, which has devastated the Middle Eastern country.

Earlier this month deputy Russian Defense Minister Yuri Borisov, was quoted as saying, “We are buying Su-57 jets for combat trials. The first stage of state trials has been concluded. … As you might imagine … many test flights must be carried out.”