The window of opportunity for diplomacy over Iran’s nuclear program “will not be opened indefinitely,” US Vice President Joe Biden warned in an interview with the German daily Süddeutsche Zeitung on Friday.

The vice president said the “burden of proof” on its nuclear program is on Tehran. “[Iran has] forfeited the confidence of the international community, and they will have to continue reckoning with crippling sanctions and increasing pressure,” Biden told the German daily.

He urged Tehran to hold diplomatic talks and even take part in direct negotiations with the US, saying it is the “time and place” for successful diplomacy.

Biden told Süddeutsche Zeitung that Iran developing a nuclear bomb is a “threat to the national security of the United States,” and that the US “will stop Iran from getting a nuclear weapon.”

The White House said on Thursday that Iran’s installation of advanced uranium enrichment machines would be a “provocative step” in further violation of United Nations resolutions against Tehran’s nuclear program.

Iran, in a letter to the International Atomic Energy Agency, said it would introduce new centrifuges to its main enrichment plant near the central town of Natanz, according to an IAEA communication to member states seen by Reuters.

Such a step could enable Iran to enrich uranium much faster than it can at the moment.

Biden set out on a European tour during which he was to meet with Syrian opposition leader Mouaz Alkhatib on Saturday to discuss US concerns about the Syrian conflict with representatives from Russia and the UN.

He was scheduled to meet with German Chancellor Angela Merkel in Berlin on Friday, French President François Hollande on Monday, and British Prime Minister David Cameron on Tuesday.

On Thursday, outgoing US Secretary of State Hillary Clinton expressed concern over increasing involvement of Iran in the Syrian conflict, including sending people and weapons to support the Assad regime.

Reuters contributed to this report.

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