The USS Abraham Lincoln Carrier Strike Group and a bomber task group was recently deployed to the Middle East early "in response to indications of heightened Iranian readiness to conduct offensive operations against USA forces and our interests", according to the Pentagon statement about the deployment.
He cautioned that the lack of attacks on Americans doesn't mean the threats have gone away.
Analysts say that provocative USA moves could be perceived by Iran as a potential imminent military threat at a time when the U.S. has already engaged in economic warfare.
The Trump administration is set to brief lawmakers Tuesday afternoon on what it alleged was evidence purporting to back its claims of a heightened threat posed by Tehran.
Shanahan said he talked to lawmakers about what the Department of Defense has been doing since May 3, when "we received credible intelligence about threats to our interests in the Middle East and to American forces". "Our biggest focus at this point is to prevent Iranian miscalculation", Shanahan told reporters on Capitol Hill.
"This is about deterrence, not about war". But if Iran attacked USA military forces first, four out of five believed the United States should respond militarily in a full or limited way, the May 17-20 poll showed. "I would consider that a hold".
He said the US' decision to counter possible threats gave the "Iranians time to recalculate".
Lawmakers are warning the Trump administration it can not take the country into war without approval from Congress, and the back-to-back briefings show the wariness among Democrats, and some Republicans, over the White House's sudden policy shifts in the Middle East.
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Rhetoric between Tehran and Washington has escalated in recent weeks as the U.S. tightened sanctions with what it said was the goal of pushing Iran to make concessions beyond the terms of a 2015 multinational deal to curb Iran's nuclear program.
Several tankers in the Gulf, including two Saudi vessels, were reportedly sabotaged as well. In a tweet later that day, however, Trump dismissed any report of his administration starting negotiations as "false" and said, "Iran will call us if and when they are ever ready".
After the Senate meeting, Republican Senator Lindsey Graham said they had told lawmakers the recent attacks on shipping and a pipeline in the Middle East were directed by the Iranian government and that US officials had detected worrisome communication between Tehran and a Shi'ite militia that prompted the military to quickly relocate an aircraft carrier.
Some House Democrats accused the administration officials of twisting intelligence to make the case for what they see as an aggressive response to any Iranian actions.
Democrats also faulted Trump's administration for, as they put it, lacking a long-term plan to deal with Iran now that Washington has withdrawn from the nuclear agreement.
US President Donald Trump's administration has ratcheted up its "maximum pressure" campaign against Iran in recent months, raising concerns that the United States and Iran are on a path to war.
"They're just hoping that in the end the Iranians decide to come to the table, without any clear plan on how that is going to happen", Democratic Senator Chris Murphy, a Foreign Relations Committee member, told reporters.