Lavrov said the United States circulated a draft that had been amended in response to French demands to other members of the six-power P5+1 for approval "literally at the last moment, when we were about to leave Geneva."
Lavrov's revelation, which has thus far been ignored by major news outlets, came in a news conference in Cairo Thursday that was largely devoted to Egypt and Syria. Lavrov provided the first real details about the circumstances under which Iran left Geneva without agreeing to the draft presented by the P5+1.
The full quote from Lavrov's press conference is available thanks to the report from Voice of Russia correspondent Ksenya Melnikova.
Lavrov noted that unlike previous meetings involving the P5+1 and Iran, "This time, the P5+1 group did not formulate any joint document."
Instead, he said, "There was an American-proposed draft, which eventually received Iran's consent." Lavrov thus confirmed the fact that the United States and Iran had reached informal agreement on a negotiating text.
He further confirmed that Russia had been consulted, along with the four other powers in the negotiations with Iran (China, France, Germany and the UK), about that draft earlier in the talks -- apparently Thursday night, from other published information.
"We vigorously supported this draft," Lavrov said. "If this document had been supported by all [members of the P5+1], it would have already been adopted. We would probably already be in the initial stages of implementing the agreements that were offered by it."
Then Lavrov revealed for the first time that the U.S. delegation had made changes in the negotiating text that had already been worked out with Iran at the insistence of France without having consulted Russia.
"But amendments to [the negotiating draft] suddenly surfaced," Lavrov said. "We did not see them. And the amended version was circulated literally at the last moment, when we were about to leave Geneva."
Lavrov implies that the Russian delegation, forced to make a quick up or down decision on the amended draft, did not realize the degree to which it was likely to cause the talks to fail.
"At first sight, the Russian delegation did not notice any significant problems in the proposed amendments," Lavrov said.
He made it clear, however, that he now considers the U.S. maneuver in getting the six powers on board a draft that had been amended with tougher language -- even if softened by U.S. drafters -- without any prior consultation with Iran to have been a diplomatic blunder.
"[N]aturally, the language of these ideas should be acceptable for all the participants in this process -- both the P5+1 group and Iran," Lavrov said.
The crucial details provided by Lavrov on the timing of the amended draft shed new light on Secretary of State John Kerry's claim in a press conference in Abu Dhabi on Monday of unity among the six powers on that draft.
"We were unified on Saturday when we presented a proposal to the Iranians." Kerry said, adding that "everybody agreed it was a fair proposal."