During the same period, the Byzantine fleet was active in the Black Sea as well: a Rus' fleet that was threatening Constantinople in 941 was destroyed by 15 hastily assembled old ships equipped with Greek fire, and the navy played an important role in the Rus'–Byzantine War of 970–971, when John I Tzimiskes (969–976) sent 300 ships to blockade the Kievan Rus' from retreating over the Danube.