Romania has warned that the strikes are part of Moscow's escalating aggression against Ukraine, while Bulgaria has stopped short of formally blaming Moscow, referring instead to drones coming from "an eastern direction."

The security concerns reached Bulgaria's shores Friday, when a local resident discovered a drone washed up near the coastal town of Shabla. The aerial vehicle appeared to have been carried ashore by the sea, said the municipality's mayor, Mariyan Zhechev. The drone's origin and whether it was connected to the recent attacks on commercial shipping have not been established.

It is unclear whether von der Leyen will be accompanied by anyone else such as European Council President António Costa or EU foreign policy chief Kaja Kallas. In Bucharest on Monday she will meet with Romanian President Nicușor Dan and Prime Minister Luca Niculescu. On Tuesday, she will meet Bulgarian Prime Minsiter Rumen Radev in Varna and visit the Nikola Vapsarov Naval Academy.

The plans come as NATO allies step up surveillance over the Black Sea following the attacks on commercial shipping. Italian, British and American reconnaissance aircraft were spotted conducting simultaneous missions over the region Tuesday, according to Bulgarian broadcaster bTV, while France has extended the deployment of a maritime patrol aircraft in Romania.

The deteriorating security situation is also prompting diplomatic efforts to protect commercial shipping. Speaking at Saturday's G20 foreign ministers' meeting in Atlanta, U.S. Secretary of State Marco Rubio said India and Turkey had put forward proposals to help grain-carrying vessels navigate the Black Sea safely amid the continuing Russia-Ukraine war.

This article has been updated with details of Ursula von der Leyen's itinerary in Romania and Bulgaria.