Three Greek-managed oil tankers were struck by drones in the Black Sea on Tuesday near Russia’s port of Novorossiysk and the Caspian Pipeline Consortium (CPC) export terminal, according to media reports.

Vessels Delta Harmony, Matilda, and Delta Supreme were hit while operating near the strategic oil hub. No serious damage or injuries have been reported. The Delta Harmony briefly caught fire but the blaze was extinguished immediately. According to initial reports, a fourth vessel, the Freud, appeared to also be hit in the attack but managing company ‌TMS denied the news.

Authorities have not yet confirmed whether the attacks involved aerial drones, sea-based drones, or a combination of both.

Earlier on Tuesday, Bloomberg reported that two tankers scheduled to load crude at the CPC terminal, Kazakhstan’s primary oil export route, had come under attack after moving offshore to await their loading slots.

Following the incident, Kazakhstan’s Energy Ministry said oil exports through the CPC terminal are continuing via Single Point Mooring 1 (SPM-1). The ministry noted that the facility was also targeted in a Ukrainian attack in late November.

Attacks on commercial shipping in the Black Sea have escalated over the past six weeks.