Ministers to reconvene in Lausanne on Wednesday morning after pushing through deadline on night of exhaustive diplomacy.

The Russian and Iranian foreign ministers have claimed there has been a significant breakthrough at nuclear talks in Lausanne, which resumed on Wednesday morning.

American diplomats denied the agreement had been finalised, saying there were still issues to resolve. But in a sign the talks could be approaching a critical moment, Barack Obama held a video-conference on Tuesday night with his national security team to be briefed on developments by the US negotiators: the secretary of state, John Kerry, and the energy secretary, Ernest Moniz.

The talks went past a midnight deadline but appeared to have gathered momentum as they approached the cutoff point.

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