Security has been upgraded at the Capitol after the U.S. drone strike that killed Iranian general Qassem Soleimani and prompted vows of retaliation from Iran, sources said.

“Never underestimate the Iranians,” one senior congressional source told Fox News. “They have a very strong presence here.”

Another source added, “We don’t know what they’re going to do,” noting that “raising the security levels to some degree is prudent.”

TRUMP ORDERS ATTACK THAT KILLS IRANIAN GEN. QASSEM SOLEIMANI

Fox News is told one area of constant concern on Capitol Hill is a potential cyberattack. The source said Iran views potential cyberwarfare “as an alternate way to attack without a bullet.”

On Friday evening, Acting Department of Homeland Security Secretary Chad Wolf tweeted: "At this time there is no specific, credible threat against the homeland."

"DHS operational components are implementing measures to enhance homeland security within their authorities and mission sets when necessary and prudent," he said.

The killing of Soleimani, the head of Iran's elite Quds Force, marks a major escalation in the standoff between Washington and Tehran, which has careened from one crisis to another since President Trump withdrew from the 2015 nuclear deal and imposed crippling sanctions.

Iran has vowed "harsh retaliation" for the U.S. strike, with the country's supreme leader, Ayatollah Ali Khamenei, warning that "harsh retaliation is waiting."

TRUMP SAYS SOLEIMANI SHOULD HAVE BEEN KILLED ‘YEARS AGO,’ WAS ‘HATED’ WITHIN IRAN

Meanwhile, President Trump accused Soleimani of planning “imminent and sinister attacks” in his first televised remarks since the deadly airstrike.

"We took action last night to stop a war,” Trump said Friday during brief remarks at his Mar-a-Lago resort in Florida. “We did not take action to start a war.”

Without divulging details about what led to the early morning airstrike that killed Soleimani and nine others, the president said the United States "caught" the general "in the act and terminated him.”

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“Soleimani made the death of innocent people his sick passion,” Trump added, saying that "what the U.S. did yesterday should have been done long ago."

The Associated Press contributed to this report.