Ukraine claimed that at least one Russian attack drone fired overnight on one of its Danube delta ports had crashed across the river on to the territory of Nato-member Romania, marking a potentially serious escalation of Moscow’s full-scale invasion of its neighbour.

Romania’s defence ministry “categorically denied” the reports on Monday. Two Ukrainian officials told the Financial Times that it was not the first time such an incident had occurred but declined to explain why Bucharest would deny it.

The drone strikes came hours before President Vladimir Putin was due to meet his Turkish counterpart Recep Tayyip Erdo?an in Sochi for talks about restarting a deal brokered by the UN and Ankara that permitted Ukrainian grain exports via its Black Sea ports.