The tense exchange took place in front of allied foreign ministers and was symptomatic of the mutual distrust between the US and many of its European allies over the war in Ukraine, Axios reported.
During a discussion on Ukraine, Kallas challenged Rubio over the US failure to escalate pressure on Russia. She reminded him of a pledge he had made at a G7 meeting a year earlier — that Washington would take more decisive action against Moscow if Russia continued to obstruct American peace efforts.
"A year has passed, and Russia hasn't moved. When will you run out of patience?" Kallas said, according to the sources.
Rubio's response was sharp. "We are doing everything in our power to end the war. If you think you can do it better, go ahead. We'll step back," he said, raising his voice, the sources told Axios.
He added that the US was trying to negotiate with both sides while only supplying one of them — Ukraine — with weapons, intelligence, and other support.
Several European ministers in the room intervened after the exchange, telling Rubio they still wanted Washington to continue its diplomatic engagement with both Russia and Ukraine, one source said. At the end of the meeting, Rubio and Kallas briefly stepped aside to try to ease the tension, two sources said.
Rubio's public denial
Speaking informally to reporters after the G7 session, Rubio denied any tension had occurred.
"These meetings are often about thanking America for the role we've played... and expressing gratitude for the mediating role we've tried to play in this war between Russia and Ukraine. Nobody there is screaming, raising their voice, or saying anything negative," he said.
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Zelenskyy dispute
Following the ministerial talks, Rubio also rejected Ukrainian President Volodymyr Zelenskyy's claim that Washington was prepared to offer Ukraine security guarantees only on condition of its withdrawal from Donetsk and Luhansk oblasts.
Zelenskyy had stated that the US was offering Kyiv security guarantees only in exchange for pulling Ukrainian forces from the unoccupied part of Donbas. Rubio called that characterisation false.
The Financial Times reported in January that US security guarantees for Ukraine would be contingent on Kyiv's prior agreement to withdraw its troops from the unoccupied part of Donbas.
