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Iran has arrested a US citizen in a move that could complicate ongoing detainee negotiations that Iran had previously described as being close to completion.

The new detainee joins three Iranian-Americans held by the Iranian government – Siamak Namazi, Emad Shargi and Morad Tahbaz – on spying charges which the US has called "baseless".

US National Security Advisor Jake Sullivan told CBS in an interview that the US had "tried very hard to secure the return of the four unjustly detained Americans in Iran."

His comments were confirmed by Semafor who stated that a fourth citizen was now part of the current detainee negotiations which are ongoing in Oman, although it did not reveal the identity of the new detainee.

The negotiations over the US citizens also involve Iran’s demands for the US to release billions of dollars in Iranian funds which are currently held in overseas banks, including in South Korea.

The arrest come as the US announced new deployments in the Gulf which are aimed at deterring a recent spate of attempted seizures of shipping in the Gulf by Iran.

The US confirmed on Thursday that it will be sending both the US Bataan amphibious readiness group and the 26th Marine Expeditional, which consists of around 2,500 marines.

They will add to a recent deployment of the destroyer USS Thomas Hudner, as well as F-35 and F-16 aircraft. A deployment ofÌý A-10 aircraft is also already patrolling the Gulf.