Outages impacting Moscow subway system’s website and mobile app on Monday have been suspected to be caused by a likely retaliatory cyberattack after its website was reported to have shown a message allegedly from Ukrainian national railway operator Ukrzaliznytsia, which had its operations disrupted by a significant cyber intrusion last week, according to The Record, a news site by cybersecurity firm Recorded Future.

Despite Russian internet regulator Roskomnadzor’s disclosure of the service interruptions, officials have yet to confirm a cyberattack. Such a development comes as Ukrzaliznytsia emphasized that the intrusion had not compromised any data after the resumption of online ticket sales, as well as app and website access. Transport infrastructure has been an attractive target for Ukrainian and Russian threat actors amid the ongoing war between both nations, with Ukraine’s IT Army hacktivist operation admitting attacks against Moscow and Kazan fare payment systems, as well as a St. Petersburg transit payment app last month.