Amidst an escalation of tensions with Iran, the US has approved the deployment of 1,000 additional troops in the Middle East for "defensive purposes" to address air, naval and ground-based threats.
The action of deploying 1,000 more troops is being taken to ensure the safety and welfare of US military personnel working throughout the region and to protect its national interests, Shanahan said on Monday.
The United States does not seek conflict with Iran, he said.
"I have authorised approximately 1,000 additional troops for defensive purposes to address air, naval and ground-based threats in the Middle East," Shanahan said.
"The recent Iranian attacks validate the reliable, credible intelligence we have received on hostile behaviour by Iranian forces and their proxy groups that threaten United States personnel and interests across the region," he said.
The US will continue to monitor the situation "diligently and make adjustments" to force levels as necessary given intelligence reporting and credible threats, Shanahan said.
Last month, National Security Adviser John Bolton said that the US was deploying the USS Abraham Lincoln Carrier Strike Group and a bomber task force to the Middle East in response to a "number of troubling and escalatory indications and warnings" from Iran.
America has blamed Iran for last week's attacks on two oil tankers in the Gulf of Oman, an accusation Tehran has denied as "baseless."
Following the Shanahan's announcement, House Speaker Nancy Pelosi demanded an immediate briefing from the Trump administration.
"Congress must be immediately briefed on the administration's decisions and plans.
"This deeply concerning decision may escalate the situation with Iran and risk serious miscalculations on either side. Diplomacy is needed to defuse tensions, therefore America must continue to consult with our allies so that we do not make the region less safe," Pelosi said.
Earlier in the day, the State Department urged Iran not to obtain a nuclear weapon, to abide by their commitments to the international community.
Earlier Iran announced that it will increase its nuclear fuel supply beyond limits set in the 2015 nuclear agreement.
"I think it's unfortunate that they've made this announcement today. It doesn't surprise anybody," State Department Spokesperson Morgan Ortagus told reporters at her news conference.
Reiterating that the Iranian nuclear deal needs to be replaced, she said Iran, as evident by its announcement and pattern of behaviour over the past few years, is keen on expanding or seems to be keen on expanding its nuclear programme, and it now wants to exceed these nuclear limits in advance of these so-called sunset clauses.
"We have made it very clear since this President came into office and since the Secretary came here that we will not tolerate Iran obtaining a nuclear weapon. Full stop.
"So any actions that they take to get a nuclear weapon will be countered by a maximum pressure campaign by the United States Government that continues to this day. There should be no relieving of sanctions for their malign and unacceptable behaviour," Ortagus said.
Senator Dianne Feinstein urged all parties to work towards a diplomatic solution to avoid further escalation.
"The Trump administration inflamed tensions by withdrawing the United States from the agreement and reimposing sanctions on Iran. Iran now says it will violate the agreement unless our European partners can make up for the economic loss caused by US sanctions," the Democratic Senator said.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)