US aircraft carrier USS Arlington joined the Abraham Lincoln aircraft carrier in the Gulf waters in a new letter to Iran.
USS Arlington carrying amphibious vehicles and fighter aircraft, and a Patriot air defense missile system.
The United States earlier moved the B-52 strategic bombers to its military bases in Qatar.
The Pentagon says the move comes in response to possible Iranian military operations against US forces in the region.
The Pentagon did not elaborate on the nature of potential Iranian threats, which Iran has categorically denied, calling the US move a "psychological war" aimed at intimidating it.