Romania's President Klaus Iohannis on Monday warned that security in the Black Sea was at risk after Russia hit Ukrainian port infrastructure on the Danube river at the border with the NATO member.
Amateur footage posted on social media by a Romanian showed an explosion just across the Danube, as drones appeared to hit the Ukrainian port of Reni on the river.
Ukrainian officials reported a four-hour Russian drone attack on port infrastructure in the southern Odesa region.
Reni is a Ukrainian port town where the Danube forms a natural frontier with Romania, and is also near the Moldovan border.
The Danube delta region, which spans Romania and Ukraine, is being used as an export route for Ukrainian grain.
US Secretary of State Antony Blinken discussed the blast with Romanian Foreign Minister Luminita Odobescu on Monday, State Department spokesman Matthew Miller told journalists in Washington.
"We will defend every inch of NATO territory. The secretary made that clear in his call with the Romanian Foreign Minister today," Miller said.
"I strongly condemn the recent Russian attacks against the Ukrainian civilian infrastructure on (the) Danube, very close to Romania," President Iohannis wrote on Twitter.
"This recent escalation poses serious risks to the security in the Black Sea. It also affects further Ukrainian grain transit, thus the global food security," he added.
Russia last week pulled out of a key deal which had allowed the safe export of Ukrainian grain through the Black Sea.
Since then Kyiv has accused Russia of targeting grain supplies and infrastructure vital to exports.
"We will continue to consult with our partners in the region on how we might find alternative ways to get the grain out of Ukraine," State Department spokesman Miller said, warning there was "no perfect solution" that could replace the deal.
Following Russia's invasion of Ukraine in February 2022, a large amount of Ukrainian grain exports have been shipped through Romania's Black Sea port of Constanta.
Romanian Prime Minister Marcel Ciolacu on Monday called on Russia to stop air strikes on Ukrainian port infrastructure.
"Romania will continue to support Ukraine in identifying practical solutions for further grain exports to global markets," he added in a press release.
The attacks have sent wheat prices soaring to their highest level for four months, Agritel analyst Arthur Portier told AFP.
"After the closure of the maritime corridor, investors are reacting to the intensification of bombardments on alternative grain export routes", he said.