Romanians will select their next president on Sunday, with far-right candidate Calin Georgescu, who has criticized NATO and praised Russian president Vladimir Putin, considered the surprising frontrunner.
On Wednesday, Romanian intelligence published claims that Moscow has sought to influence the outcome using "considerable resources" to promote Georgescu's campaign on social media.
It also said that official election websites were targeted by 85,000 cyber-attacks around the first round of the elections in November.
Georgescu's opponent in Sunday's runoff is the pro-European centrist, Elena Lasconi.
The U.S. Department of State said that it was concerned by the "report of Russian involvement in malign cyber activity" and warned that Romania should not step away from NATO and the European Union.
"Romania’s hard-earned progress anchoring itself in the Transatlantic community cannot be turned back by foreign actors seeking to shift Romania’s foreign policy away from its Western alliances," it said in a statement.
"Any such change would have serious negative impacts on U.S. security cooperation with Romania, while a decision to restrict foreign investment would discourage U.S. companies from continuing to invest in Romania."
Russia has denied interfering in the Romanian elections, but intelligence services in Bucharest have accused it of a "massive" hybrid attack.
One unclassified Romanian intelligence document, cited by Reuters, alleged that Georgescu, who has declared zero funds spent in his campaign, was promoted on social media platform TikTok through coordinated accounts and paid advertising.
The report also said that the scale of cyber-attacks in November suggested that a foreign state was behind them.
"The attacks continued intensively including on election day and the night after elections," one document in the report said.
"The operating mode and the amplitude of the campaign leads us to conclude the attacker has considerable resources specific to an attacking state."
Georgescu, who was considered a fringe candidate before his shock victory in the first round, wants to end his country's support for neighboring Ukraine in its war against Russia.
He has also pledged to ban the export of Ukrainian grain through Romanian territory.
His rival, Lasconi, who is backed by Romania's social democrat prime minister Marcel Ciolacu, has portrayed the election as a choice between "NATO protection and Putin's war".
