The United States is sending additional troops to the Middle East in response to mounting concerns over Iran, which have escalated since attacks on two oil tankers in the Gulf of Oman.
The US military on Monday released images that it says bolster its claim Iran is behind the attacks - but Iran continues to deny responsibility.
The new US deployment to the Middle East would be in addition to a 1,500 troop increase announced last month in response to tanker attacks in May that the US also blamed on Iran.
The standoff comes more than a year after President Donald Trump announced Washington was withdrawing from a 2015 nuclear deal.
Iran said on Monday it would soon breach limits on how much enriched uranium it can stockpile under the deal, which a White House National Security Council spokesman described as "nuclear blackmail."
The 2015 accord, which Iran and the other signatories have maintained following the US withdrawal , caps Iran's stock of low-enriched uranium at 300 kg enriched to 3.67 per cent.
But Iran's Atomic Energy Organisation spokesman Behrouz Kamalvandi said on Monday: "We have quadrupled the rate of enrichment (of uranium) and even increased it more recently, so that in 10 days it will bypass the 300 kg limit.""
The move further undermines the nuclear pact also signed by Russia, Britain, Germany, China and the European Union, but Iranian President Hassan Rouhani said the collapse of the deal would not be in the interests of the region or the world.
Iran's Rouhani said on Monday that European nations still had time to save the accord.
"It's a crucial moment, and France can still work with other signatories of the deal and play an historic role to save the deal in this very short time," Rouhani was quoted as saying during a meeting with France's new ambassador in Iran.
Meanwhile US Secretary of State Mike Pompeo has spoken to officials from the North Atlantic Treaty Organisation, China, Kuwait, South Korea, Britain and other countries to share its evidence of Iran's involvement in the attacks on the Norwegian and Japanese tankers.
Pompeo said on Sunday the United States did not want to go to war with Iran but would take every action necessary, including diplomacy, to guarantee safe navigation through Middle East shipping lanes.
