Moscow has repeatedly attacked Ukraine's ports in the south, including areas bordering Romania, since exiting a grain deal that aimed to protect grain exports through the Black Sea this summer.

According to the defence ministry, a crater was found in an uninhabited area close to the town of Grindu in Tulcea county, which faces the port of Reni in southern Ukraine across the Danube.

The crater was the "result of (an) uncontrolled crash of a drone used in the attack on Ukrainian port infrastructure," the statement said.

In response to the "new violation of Romania's airspace, contrary to international law", the head of Russia's diplomatic mission will be summoned, the Romanian Ministry of Foreign Affairs said in a statement.

The ministry said it strongly condemns Moscow's "repeated attacks" and consultations with "NATO allies on developments at the Romanian-Ukrainian border, including today's incident" are underway.

Romania's army also spotted the drone on its radar surveillance system that "indicated a possible unauthorised entry into national airspace, with a signal detected at low altitude en route to Grindu," it added.

F-16 and Eurofighter Typhoon fighter jets were also deployed to monitor the airspace.

In September and October, drone debris was discovered in Romania near its border with Ukraine, but officials have stressed that the country had not been intentionally targeted.

As a result, measures to protect the population near the border with Ukraine have been taken, including the construction of air-raid shelters in the eastern Romanian village of Plauru.

Ukraine said Thursday that its air-defence systems had destroyed more than 40 Iranian-designed drones launched by Russian forces targeting the southern city of Odesa.