Basseterre, St. Kitts and Nevis - Iran is seeking to develop intercontinental ballistic missiles that could one day reach the US, Secretary of State Marco Rubio claimed on Wednesday as he continued to agitate for war.

The US had purportedly observed Iran extending the range of missiles already in its arsenal, Rubio told reporters in Basseterre, the capital of St. Kitts and Nevis.

Offering no evidence for his claims, he insisted it was clear that Iran is on a path to one day being able to develop weapons that could reach the US.

"This is an unsustainable threat," Rubio, who also serves as President Donald Trump's national security adviser, said.

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Iran already possesses weapons capable of threatening large parts of Europe, he warned, while admitting to being surprised that Tehran continues to invest in longer-range missiles despite sanctions and severe economic difficulties.

Speaking ahead of another round of talks between the US and Iran in Geneva, Rubio said diplomacy was never off the table and that Trump preferred peaceful solutions. He described Thursday's meeting – the third round of negotiations this year – as the "next opportunity to talk."

According to Rubio, the indirect talks, mediated by Oman, will focus primarily on Iran's disputed nuclear program, which Trump repeatedly claimed was completely obliterated by unprovoked and unprecedented attacks he ordered last summer. The US delegation will be led by special envoy Steve Witkoff.

Rubio said Washington hoped progress could be made, but stressed that Iran's refusal to discuss ballistic missiles – an issue strongly pushed by Israel – remained "a big problem."

Iran has stressed again and again that it is open to negotiations, but will not be threatened into capitulating to every US demand, in particular when it comes to its defensive military capabilities.