US Secretary of State Marco Rubio on Tuesday briefed the so-called Gang of Eight, the bipartisan group of top congressional leaders, to discuss Iran.
Senate minority leader Chuck Schumer, a Democrat, said after the meeting that "this is serious" and that the Trump administration needs to "make its case to the American people", without elaborating.
Senate Republicans issued a warning to Iran after the meeting with Mr Rubio, who is also acting National Security Adviser.
"Iran’s leaders know the consequences for pursuing nuclear weapons," they said in a post on X. "They should not repeat past mistakes."
US media reported that CIA Director John Ratcliffe was also present.
The briefing came hours before President Donald Trump was set to deliver his State of the Union address – a speech already overshadowed by rising tension with Tehran over its nuclear programme.
Despite continuing negotiations, the likelihood of war seems only to get stronger by the day as the US builds up its military arsenal in the region.
The US wants Tehran to end all uranium enrichment, an apparent red line for Iran, which insists its nuclear programme is for peaceful purposes. It says it could agree to dilute its production if international economic sanctions are lifted.
Meanwhile, the Trump administration has sent a second US aircraft carrier, the USS Gerald Ford — the largest vessel of its kind — to the region. Military experts say it is prepared to launch a strike on Iran if the President authorises one.
In 2011, then-president Barack Obama briefed the Gang of Eight before the raid in Abbottabad, Pakistan that killed Osama bin Laden.
The last time the Trump administration briefed the group was on January 5, after the capture of Venezuelan President Nicolas Maduro.
The group was not notified before the US strike on three of Iran's nuclear sites in June last year, in an operation called Midnight Hammer.
"You'll hear the President tout the accomplishments overseas in the United States military over the past year under his leadership, one of which, of course, was Operation Midnight Hammer," White House press secretary Karoline Leavitt told reporters on Tuesday.
"You'll hear the President proudly and rightfully say that the United States military is the strongest and most meaningful fighting police."
The Gang of Eight consists of Mr Schumer, John Thune, Mike Johnson, Hakeem Jeffries, Tom Cotton, Mark Warner, Rick Crawford and Jim Himes.
"I think it is incumbent upon the President to make the case of what our country's goals are, what our country's interests are, and how we're going to protect American interests in the region," said Mr Warner, a Democratic senator from Virginia who is also vice chairman of the Senate intelligence committee.
"Maybe we’ll hear that tonight, but if we don’t hear it tonight, we need to hear it very, very, very soon."
Democrats in Congress are concerned that Mr Trump will ignore the War Powers Act, which is legislation requiring congressional authorisation for significant military engagements.
Under the "America First" slogan, he campaigned on a promise to withdraw the US from involvement in conflicts abroad, calling it a political liability and a wasteful expense for American taxpayers.
