Budapest, Hungary - Hungary is purchasing crude oil worth $500 million from the US to "diversify supply sources," a government official said on Thursday, following the visit of US Vice President JD Vance.

Vance visited Budapest this week to throw his support behind nationalist Prime Minister Viktor Orban, who is facing an unprecedented challenge to his 16-year rule in Sunday's parliamentary elections.

Hungarian oil company MOL will be purchasing crude oil from the US in the "coming weeks and months," Orban's chief of staff, Gergely Gulyas, told a press briefing.

"This step serves to diversify supply sources and is part of strengthening US–Hungarian energy relations," he added.

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MOL is purchasing 510,000 tons of crude oil worth $500 million from the US and American energy companies, the White House said in a statement.

The statement outlined "achievements" in the two countries' ties, reinforced by Vance's "historic trip" to the European Union country.

US President Donald Trump has endorsed Orban's re-election bid.

Secretary of State Marco Rubio also visited the central European country of 9.5 million people in mid-February.

Hungary has continued to rely heavily on Russian energy.

Orban broke ranks with most other EU leaders to maintain close ties with Russian President Vladimir Putin despite Moscow's invasion of Ukraine.