Top US official refuses to say if Trump has authority to strike Iran after Pompeo lays out 'red line' Secretary Pompeo said recently the red line for US strikes was one American dead

President Donald Trump's top envoy for Iran refused to say whether the administration believes it has the authority to strike Iran militarily, amid heightened tensions with the country over its threats to expand its nuclear fuel stockpiles and its reported attacks on oil supply chains in the Middle East.

While Democrats in Congress are concerned about Trump's legal authority, Secretary of State Mike Pompeo told Iraqi leaders last month that the administration's "red line" for military action against Iran would be the killing of an American citizen -- something Trump allies in Congress said would not require the legislature's approval.

The debate over military strikes comes after a series of attacks in the Middle East that the U.S. has blamed on Iran, including two oil vessels that were -- according to U.S. officials -- attacked with mines by Iran's Islamic Revolutionary Guard Corps. Four oil vessels suffered similar sabotage in May while also transiting near the Strait of Hormuz, a vital shipping waterway off the coast of Iran.

On Tuesday, a rocket attack outside Basra in southern Iraq blasted workers' sleeping quarters at an oil drilling site operated by ExxonMobil and other international firms. Three Iraqi workers were wounded and a second rocket damaged a Turkish oil facility, but caused no injuries. No one has claimed responsibility, and the U.S. -- so far -- has not pointed the finger at Iran.

Iran denies responsibility for nearly all of these incidents, but analysts say elements within the country are lashing out to protest U.S. sanctions and drive up the price of oil, to help ease the economic pain of Trump's "maximum pressure" campaign. Trump reimposed sanctions on Iran's oil industry, central bank and much more after withdrawing from the Iran nuclear deal, which lifted those sanctions in exchange for limits on and inspections of Iran's nuclear program.

Without those economic benefits, Iran threatened on Monday to break the deal and enrich uranium at levels higher than allowed, pressuring European countries to come to its economic rescue.

"Our pressure campaign is working," U.S. Special Representative for Iran Brian Hook told the House Foreign Affairs Committee on Wednesday, adding that the administration's intense enforcement -- with no waivers for oil purchases, for example -- has cost Iran billions of dollars.

But lawmakers are concerned that despite those budget shortfalls, Tehran is responding to this pressure with more aggressive action of its own. At the risk of stumbling into a heightened conflict, House Democrats pressed Hook on whether the administration believes it has authority to strike Iran under the 2001 Authorization for Use of Military Force.

Passed three days after the Sept. 11, 2001 attacks, the AUMF provides the executive branch with broad authority to use force "against those nations, organizations, or persons" that it determines "planned, authorized, committed, or aided the terrorist attacks" on Sept. 11, "or harbored such organizations or persons."

There have been growing concerns that the administration may cite connections between Iran and al-Qaeda to justify military strikes, even though some doubt how strong those ties currently are.

When asked repeatedly about that, Hook declined to answer, instead telling Rep. Ted Deutch, D-Fla., "If the use of military force is necessary to defend U.S. national security interest, we will do everything that we are required to do with respect to congressional war powers, and we will comply with the law."

Hook was also asked whether the administration has the power to declare war and if he had read the Constitution, which gives Congress the authority to declare war, but gives the president certain powers as the commander-in-chief.

He said he had read the Constitution, but Rep. Ted Lieu, D-Calif., who had asked, cut off the line of questioning and submitted the Constitution to the hearing's record.

Hours later, the House voted in favor of an amendment to repeal the AUMF as part of a defense spending package, although the bill's chances in the Senate are slim.

Questions around the AUMF, however, may be beside the point, in particular if Iran or a proxy force attacks an American. A State Department official confirmed to ABC News that while on a surprise visit to Baghdad last month, Pompeo told Iraqi leaders that the death of one U.S. citizen by Iran or any of its associated forces would spark a U.S. counterattack -- a message he wanted them to convey to their neighbor and ally Iran.

Hook suggested as much to Congress, saying, "Everything that we are trying to do now is defensive. ... There is no talk of offensive action."

Trump's allies in Congress agree.

"The AUMF is irrelevant. You don't need an AUMF to respond to an attack. I don't know why people are confused," Sen. Marco Rubio, R-Fla., told ABC News.

It's unclear if the administration would go further, as some Senate Republicans have suggested. Sen. Tom Cotton, R-Ark., said on Sunday that last week's attacks "warrant a retaliatory military strike," while Sen. Lindsey Graham, R-S.C., said Tuesday if Iran continues to disrupt oil supplies, "We should put their refineries on the target list ... I'm talking about blowing it up."

But Hook repeated what Trump and Pompeo have said -- that the administration does not seek war. Instead, he said, they hope their maximum pressure campaign drives Iran to the negotiating table, adding that the U.S. intelligence community assessed this is the strongest way to convince Iran to negotiate.

On Wednesday, Hook, acting Assistant Secretary of Defense for International Security Affairs Kathryn Wheelbarger and an unnamed intelligence officer for Iran briefed senators in a classified setting.

After that briefing, Hook departed for the Middle East to meet with leaders in Saudi Arabia, the United Arab Emirates, Kuwait, Oman and Bahrain, where he will work to bolster Arab allies' support for the U.S. He will next travel to Paris to meet with the United Kingdom, Germany and France on the same day that Iran has promised it will begin making good on its threat to enrich more uranium.

ABC News's Mariam Khan contributed to this report.