US Vice President Joe Biden has said that there was a less than even shot of a nuclear deal with Iran but that it was still worth pursuing.
Mr Biden, speaking at a forum on the Middle East at the Brookings Institution think tank, rejected calls for more sanctions against Tehran over its nuclear programme.
This is because this is not the time to risk a breakdown when we still have a chance for a breakthrough.
Mr Biden said the sanctions imposed by the US and allies were working by slowing Iran's economy, as well as its nuclear programme.
It's frozen the programme, it has given us a shot for a peaceful solution, he said.
Mr Biden said that while the United States and Israel have tactical disagreements, Washington remained fully dedicated to Israel's security.
He said: "we will not let Iran acquire a nuclear weapon - End of discussion. Not on our watch."
Last month Iran said it would not make excessive concessions on its nuclear programme, which Tehran says is for energy-creation purposes, as part of a 12-year dispute.
Mr Biden also spoke about the relationship between Israel and the United States, saying there was no daylight between the administration and the Israeli government's position on Israel's security.
However, Mr Biden did criticise Israel for expanding its illegal Jewish settlements in Arab East Jerusalem and the West Bank as well as the demolition of homes of relatives of Palestinian attackers, saying it risks stroking tensions even further.