WASHINGTON >> As questions mount over President Donald Trump’s tough talk on Iran, top national security officials are heading to Capitol Hill to brief Congress. But skeptical Democrats have asked for a second opinion.

The competing closed-door sessions Tuesday, unusual and potentially polarizing, come after weeks of escalating tensions in the Persian Gulf that have raised alarms over a possible military confrontation with Iran. Lawmakers are warning the Trump administration it cannot 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.

Trump, veering between bombast and conciliation in his quest to contain Iran, threatened today to meet provocations by Iran with “great force,” but also said he’s willing to negotiate.

“We’ll see what happens,” Trump told reporters today as he left the White House for a campaign rally. He said Iran has been “very hostile.”

“We have no indication that anything’s happened or will happened, but if it does, it will be met, obviously, with great force,” Trump said. “We’ll have no choice.”

Trump said while there are no talks with Iran he still wants to hear from them, “if they’re ready.”

Over the past several weeks the U.S. has sent an aircraft carrier and other resources to the Persian Gulf region, and evacuated non-essential personnel from Iraq, amid unspecified threats the administration says are linked to Iran.

The administration is sending Secretary of State Mike Pompeo, Acting Defense Secretary Patrick Shanahan and other top brass, including Gen. Joseph Dunford, the chairman of the Joint Chiefs of staff, for closed-door briefings Tuesday with both the House and Senate.

But House Democrats, deeply skeptical of the information from the Trump officials — and mindful of the drumbeat of claims during the run-up to the Iraq War — invited former CIA Director John Brennan and former State Department official Wendy Sherman, who negotiated the Iran nuclear deal.

Brennan, an outspoken Trump critic, does not have a formal briefing planned but is prepared to answer questions on Iran — and is willing to do the same for Republicans, said a person familiar with the matter who was not authorized to discuss it publicly. The intent, the person said, is to provide information and not to be partisan.

Top Democrats say Trump escalated problems by abruptly withdrawing the U.S. from the Iran nuclear deal, a complex accord negotiated during the Obama administration to prevent the country from nuclear weapons production.

Trump’s allies in Congress, including GOP Sen. Lindsey Graham of South Carolina, say the threats from Iran are real. Graham urged Trump to “stand firm” and said he received his own briefing over the weekend from John Bolton, Trump’s national security adviser.

“It is clear that over the last several weeks Iran has attacked pipelines and ships of other nations and created threat streams against American interests in Iraq,” Graham tweeted. “If the Iranian threats against American personnel and interests are activated we must deliver an overwhelming military response.”

But Democratic Rep. Ruben Gallego of Arizona, an Iraq War veteran, tweeted that after having received “the same” intelligence briefing, that was not his conclusion.

“That is not what is being said. This is total information bias to draw the conclusion he wants for himself and the media,” Gallego tweeted.

Sen. Chris Murphy, D-Conn., said it’s important to more fully understand the situation. “I think Iranians think that our moves are offensive, we think their moves are offensive, that’s how you get into wars by mistake,” he said.

Graham’s reference to Iran having attacked ships appeared to be a further indication that the U.S. military has concluded that Iran was behind the reported attack May 12 on four commercial vessels off the coast of the United Arab Emirates.

At the outset of an investigation into those apparent attacks, which damaged vessels of Saudi Arabia, the UAE and Norway but caused no injuries, U.S. officials had said they appeared to be carried out by Iran.

A U.S. official said today the probe was finished and evidence still pointed at Iran, although the official did not provide details. The official was not authorized to publicly discuss the matter and so spoke on condition of anonymity.

On Sunday, a rocket landed near the U.S. Embassy in the Green Zone of Iraq’s capital of Baghdad, days after nonessential U.S. staff were ordered to evacuate from diplomatic posts in the country. No one was reported injured. Iraqi military spokesman Brig. Gen. Yahya Rasoul told The Associated Press that the rocket was believed to have been fired from eastern Baghdad, an area home to Iran-backed Shiite militias.

Defense officials said no additional Iranian threats or incidents had emerged in the days since the USS Abraham Lincoln carrier battle group arrived in the Arabian Sea late last week.

Iran, meanwhile, announced that it has quadrupled its uranium-enrichment production capacity. Iranian officials made a point to stress that the uranium would be enriched only to the 3.67% limit set under the 2015 nuclear deal with world powers, making it usable for a power plant but far below what’s needed for an atomic weapon.

The state-run IRNA news agency quoted Behrouz Kamalvandi, the spokesman of the Atomic Energy Organization of Iran, as acknowledging that capacity had been quadrupled. He said Iran took this step because the U.S. had ended a program allowing it to exchange enriched uranium to Russia for unprocessed yellowcake uranium, as well as ending the sale of heavy water to Oman. Heavy water helps cool reactors producing plutonium that can be used in nuclear weapons.

Tehran long has insisted it does not seek nuclear weapons, though the West fears its program could allow it to build them.

Trump’s remarks reflect what has been a strategy of alternating tough talk with more conciliatory statements, which he says is aimed at keeping Iran guessing at the administration’s intentions.

He described his approach in a speech Friday, saying, “It’s probably a good thing because they’re saying, ‘Man, I don’t know where these people are coming from,’ right?”