The US sent a second bomber and Patriot air defence missile system to the Middle East on Friday, adding to the bombers ordered by the White House to counter threats from Iran that have started to arrive at America's main air base in the region.
In a statement issued on Friday, the US Department of Defense said it had deployed USS Arlington, a San Antonio-class ship that transports US Marines, amphibious vehicles, conventional landing craft and rotary aircraft with the capability of being used for special missions.
"These assets will join the USS Abraham Lincoln Carrier Strike Group and a US Air Force bomber task force in the Middle East region in response to indications of heightened Iranian readiness to conduct offensive operations against U.S. forces and our interests," it said.
"The Department of Defense continues to closely monitor the activities of the Iranian regime, their military and proxies. Due to operational security, we will not discuss timelines or location of forces."
The Pentagon added that it "does not seek conflict with Iran".
The US Air Force released photos showing B-52H Stratofortress bombers arriving at Al Udeid Air Base in Qatar on Thursday. Others landed at an undisclosed location Wednesday.
The air force identified the aircraft as coming from the 20th Bomb Squadron of Barksdale Air Force Base in Louisiana.
President Donald Trump's security adviser John Bolton announced on Sunday that the US was sending the USS Abraham Lincoln aircraft carrier strike group and bombers to counter "credible threats" from Iran or its proxies. The Abraham Lincoln crossed the Suez Canal on its way to the Arabian Gulf waters on Thursday.
US Secretary of State Mike Pompeo said on Thursday that Iran had "engaged in an escalating series of threatening actions and statements in recent weeks".
He said the US would give a "swift and decisive" response to any attacks against US interests or citizens.
Tensions between Washington and Tehran have escalated since Mr Trump pulled the US out of the 2015 nuclear deal a year ago and re-imposed sanctions on Iranian oil exports and other sources of revenue.
The Trump administration says it is seeking diplomatic engagement with Tehran to try to negotiate a new nuclear deal and address Iran’s regional policies.
CNN quoting a diplomatic source reported on Friday that the White House contacted the Swiss government on Thursday to share a phone number the Iranians could call US President Donald Trump on.
"I'd like to see them call me," Mr Trump told reporters in reference to Iranian leaders on Thursday.
The source added that the “Swiss likely won't hand over the number unless the Iranians specifically ask for it and it's thought they are highly unlikely to do so.”