Last year, Russia made 15 official "red line" statements, compared with 24 it made in the first year of its invasion, the Kyiv Post reported. Putin has said that the threat of nuclear action could happen if the red line of an existential threat to the state was crossed, although what this is has not been clearly defined.

Ukraine has said it destroyed a Russian fighter jet and ammunition warehouse in a series of strikes over the weekend.

Ukraine's military says it attacked and destroyed a Russian submarine while it was anchored at a port in the occupied Crimean peninsula. The Rostov-on-Don, a kilo-class attack submarine launched in 2014,