U.S. officials have said they believe Iranian combat divers were behind the attacks on four oil tankers near the Persian Gulf over the weekend, and they tell CBS News senior national security correspondent David Martin there's still no sign Iran is backing off purported plans to attack Americans in the region.

Martin said American officials have him there is "credible" and "urgent" intelligence that Iran has ordered Shiite militias in Iraq to prepare to conduct attacks against U.S. troops and diplomats in the country. 

On Wednesday the State Department ordered all non-emergency staff and their families to leave Iraq, a nation on Iran's southern border in which the Iranian government backs various militia groups which have fought U.S. troops before.

"U.S. citizens in Iraq are at high risk for violence and kidnapping. Numerous terrorist and insurgent groups are active in Iraq and regularly attack both Iraqi security forces and civilians. Anti-U.S. sectarian militias may also threaten U.S. citizens and Western companies throughout Iraq," the State Department said in its advisory

Secretary of State Mike Pompeo renewed the Trump administration's warning on Tuesday that the U.S. would retaliate against Iran if it does attack American interests in the Middle East, but he declined to pin the blame for the tanker sabotage on Tehran.

He said he didn't have anything "concrete about the connection" between Tehran and the tanker attacks, adding: "I think in the coming hours and days we'll know the answer to that."

At a campaign rally on Tuesday evening, President Trump emphasized what is becoming one of the hallmarks of his hardline foreign policy, telling supporters that his administration was "holding dangerous regimes accountable by denying them oil revenue to fund their corruption, oppression and terror."

But as Martin reports, while the U.S. has put a stranglehold on Iran's economy, the country remains dangerous.

U.S. officials told Martin it was highly likely that Iran's Islamic Revolutionary Guards were responsible for Sunday's attacks that blew holes in the hulls of Saudi and Norwegian tankers anchored off the Emirati port of Fujairah, just outside the Persian Gulf.

Iranian combat divers are believed to have attached explosives to the ships' hulls, but a defense official told CBS News that further investigation was still needed.