Secretary of State Mike Pompeo has been secretly delivering a message to Iran warning that the death of even a single American service member would trigger a military counterattack, The Washington Post reported Tuesday evening.

Pompeo and White House national security adviser John Bolton are leading the charge on the "maximum pressure" strategy directed at Iran.

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As the US military builds up its forces in the Middle East, America's top diplomat has been privately warning the Iranians that the death of even a single US service member at the hands of Iran or one of its proxies would trigger a military response, The Washington Post reported on Tuesday, citing US officials.

In May, the US detected signs of possible Iranian aggression targeting US forces and interests in the Middle East. The US responded by deploying the USS Abraham Lincoln carrier strike group and a bomber task force to the US Central Command area of responsibility.

Read more: The US is sending a ton of firepower to take on Iran — here's everything headed its way

White House national security adviser John Bolton issued a statement on May 5 saying that the military assets deployed to the region were meant "to send a clear and unmistakable message to the Iranian regime that any attack on United States interests or on those of our allies will be met with unrelenting force."

Two days later, Secretary of State Mike Pompeo made an unscheduled trip to Baghdad, where he delivered the warning that one American fatality would be enough to trigger a counterattack, The Post reported. Pompeo, a former US Army officer, has been a major player, together with Bolton, in shaping the US "maximum pressure" strategy directed at Iran.

More US military assets have since been moved into the region, and more are on the way in the wake of suspected limpet mine attacks on tankers that the US blames on Iran. US military leaders revealed on Tuesday that the US does not plan to carry out a unilateral military response to the tanker attacks.

Read more: The US is sending 1,000 more troops to the Middle East to confront Iran after the recent tanker attacks

Gen. Paul Selva, the vice chairman of the Joint Chiefs of Staff, said any military action taken in response to the tanker attacks would "require an international consensus," something the US military has been trying to secure through the release of evidence it says points to Iran's culpability.

Read more: The US military is building a case against Iran with clearer images from the latest tanker attacks

"If the Iranians come after US citizens, US assets or [the] US military, we reserve the right to respond with a military action, and they need to know that," the country's second-highest-ranking general told reporters. "The Iranians believe that we won't respond, and that's why we've been very clear in our message."

Iran is "lashing out against the international community," but the Iranians "haven't touched an American asset in any overt attack that we can link directly to them," he added.

Read more: An Iranian boat reportedly fired a missile at a US drone right before the tanker attack

"What happens if Americans are killed? That changes the whole thing," a senior Trump administration official told The Washington Post. "It changes everything."

Pompeo, who appears to be taking the lead on the standoff with Iran amid a reshuffling of senior leadership at the Pentagon, visited US Central Command on Tuesday, the same day acting Defense Secretary Patrick Shanahan withdrew his name from the nomination for defense secretary and said he would be stepping down.

"We are there to deter aggression. President Trump does not want war," Pompeo said. "We will continue to communicate that message while doing the things that are necessary to protect American interests in the region."