US lawmakers responded to reports that the Pentagon is preparing for weeks of ground operations in Iran, as thousands of US troops assemble in the Middle East and the conflict showed signs of entering a new, more dangerous phase.

Officials told the Washington Post that a ground operation in Iran could be limited to raids by Special Operations forces and infantry troops, but it was unclear whether Donald Trump would approve any of the Pentagon’s plans.

Senator James Lankford, a Republican, told NBC’s Meet the Press that he had not ruled out supporting troops on the ground but that “we’ve got to be able to know what the objectives are and what they’re actually carrying out.”

Lankford, who serves on the Senate’s committee on intelligence, said it was important to “finish” the job but know “what boots we’re putting on the ground”.

“If this is special forces to be able to carry out a specific operation – get in, get out – that’s very different than long-standing occupation,” he said. “The worst thing that can happen is to be able to have this kind of conflict start and to not end it, to leave it undone”.

“We’ve got to be able to finish this,” he added.

Responding to the Washington Post report, White House press secretary Karoline Leavitt said: “It’s the job of the Pentagon to make preparations in order to give the commander-in-chief maximum optionality. It does not mean the President has made a decision.”

A further 3,500 US soldiers and marines arrived in the Middle East on Sunday as part of a unit led by the warship USS Tripoli, which also includes assault and transport assets. The US typically stations around 50,000 troops in the region.

Options for use of the military build up include plans aimed at securing the strait of Hormuz, operations to seize Iran’s highly enriched uranium or seizing Iranian oil facilities.

Trump has previously said he was “not putting troops anywhere” amid apparent splits in his Maga base over foreign military engagements and the need for congressional approval.

On Sunday, Lankford was asked if the president needed congressional approval to deploy US troops in Iran. Lankford demurred, saying it was “contingent” on how they are used.

“If we had a long-standing war that’s happening, go back again to what happened in Iraq or in Afghanistan, yes,” Lankford said. “If this is to protect Americans and to be able to make sure that we’re in there for a season and we’re stopping and getting out, that’s very, very different. So again, this is all contingent.”

Senate Republicans have previously rejected multiple war powers resolutions aimed at limiting Trump’s ability to launch further military action against Iran without congressional approval.

The Pentagon has reportedly requested an additional $200bn for the war, in addition to its annual $1tn budget. Trump has said the additional funds were being requested “for a lot of reasons, beyond even what we’re talking about in Iran”.

That comes as an Iranian missile strike destroyed a US E-3 Sentry early warning and control aircraft on the ground at Prince Sultan air base in Saudi Arabia. The loss of the $300m plane is the first known combat incident for the type. The US is believed to have about eight in service.

Lankford indicated that if the request comes, Congress “will have to speak at that moment to be able to talk about how far, what the plans are, what we’re going to do”.

The House majority leader, Steve Scalise, said on Sunday that the Trump administration had been meeting its objectives in the conflict.

“The whole world knows that a nuclear armed Iran would have been a danger to the world,” he told ABC News. “Just look at what Iran is doing right now. They’ve actually united, not only Israel, but all the other Arab nations around them against Iran, because of the danger that they pose.”

Scalise rejected any characterization that the administration has not laid out its objectives. He said Trump “understands what needs to be done” and has “a great team around him.”

Democrats reacted on Sunday to signs that the conflict could be entering a more dangerous phase.

New Jersey senator Cory Booker said that the Trump administration had “gotten us into what will be looked at as one of the greatest blunders, presidential blunders of our time”.

Booker said that by not seeking congressional approval, Trump was “pushing us further and further into a conflict with no foreseeable off-ramp and thousands of more troops moving into that region”.

He said that the US military engagement with Iran “is clearly not just a war, but the biggest military engagement we’ve had since the war in Afghanistan” and questioned its planning.

“This has been the problem from the start,” Booker said, adding that Trump did not “make his case to us or the American people or strategic allies in the region”.

Maryland senator Chris Van Hollen told ABC’s This Week he hoped an additional Pentagon budget request would not pass in Congress.

“I don’t think we should be providing more money for an illegal war of choice to a president who promised during the campaign that he would not drag America into new wars, especially in the Middle East and a war that is now making us less, not more safe and has already cost American lives, is costing billions of dollars every day, oil and gas prices are going up.”

Van Hollen added that a president who campaigned on lowering prices and ending foreign wars “has started foreign wars along with Prime Minister Netanyahu and prices are going through the roof. So no, we should not keep funding an illegal war of choice that’s making us less safe.”

Iran’s parliament speaker Mohammad Bagher Ghalibaf on Sunday warned the US against a ground invasion, threatening to set the American troops “on fire” and step up attacks on US allies.

According to Iranian official media, Ghalibaf said Iranian forces “are waiting for the arrival of American troops on the ground to set them on fire and punish their regional partners forever”.

Iran policy analyst Karim Sadjadpour told CBS’s Face the Nation that he doesn’t “see any possibility of a resolution to this conflict” outside of a negotiated settlement.

“I think the US and Iran are miles apart when it comes to their goals here,” he warned, adding: “I think we could see potential ceasefire that opens the strait of Hormuz, which would shift us back from a hot war, back to a cold war.”

Iran’s Revolutionary Guard said it was prepared to target US universities in the Middle East in retaliation for what it claimed were US-Israeli strikes had destroyed two Iranian universities.

“If the US government wants its universities in the region to be free from retaliation… it must condemn the bombing of the universities in an official statement by 12 noon on Monday, March 30, Tehran time,” said the statement published by Iranian media.