President Joe Biden faced intense political pressure Monday to retaliate against Iran for a deadly drone strike on US troops, posing a major challenge for the Democrat in an election year.

Striking Iran would dramatically escalate the risk of the wider war Biden says he's trying to avoid -- not to mention the possibility of more US caskets coming home in the months before polls open.

But with Republicans urging the 81-year-old to hit Iran directly, Biden can ill afford to portray weakness as he struggles with low approval ratings ahead of a likely rematch with former president Donald Trump.

"He's under tremendous pressure -- the administration's in a kind of a lose-lose situation," Colin Clarke, research director at the Soufan Center in New York, told AFP.

"I think he's going to get hammered by people saying he's weak and he's going get hammered by people saying he's going too far. So it's damned if you do, damned if you don't."

The White House said Monday that Biden was "weighing his options" for a "very consequential" answer to the attack on a base in Jordan that killed three US troops, the first to die in hostile action since the start of the Israel-Hamas war on October 7.

Biden himself said "we shall respond" during a campaign event at a church in South Carolina on Sunday, one of a series he's held in recent days as he seeks to kickstart his bid for a second term.

Iran has denied any link to the attack, which Biden blamed on Iran-backed militias.

National Security Council spokesman John Kirby strongly denied that Biden's decision would be influenced by the election.

"He's not looking at political calculations, or the polling or the electoral calendar as he works to protect our troops ashore and our ships at sea," he told reporters.

"And any suggestion to the contrary is offensive."

'Hit Iran now'

But the issue has become a political weapon for Republicans -- and Trump in particular, as he seeks a return to the White House in November's election and vengeance for his 2020 loss to Biden.

Trump described the deaths as a "consequence of Joe Biden's weakness and surrender" -- focusing on a deal the Biden administration made with Iran last year to free US captives in exchange for unblocking $6 billion in Iranian funds.

He could also point to the fact that he had ordered the US strike that killed Iranian Revolutionary Guards general Qasem Soleimani in 2020 -- although Trump has also admitted to calling off another strike on Iran with 10 minutes to spare in 2019 despite being "cocked and loaded."

Other Republicans also gave notice they would use Iran as a test case of Biden's strength ahead of elections.

"The entire world now watches for signs that the President is finally prepared to exercise American strength," Republican Senate Minority Leader Mitch McConnell said.

Lindsey Graham, the top Republican on the Senate Judiciary Committee and a leading hawk, urged Biden to "hit Iran now. Hit them hard."

The dilemmas facing Biden are huge though.

Direct strikes on Iranian territory would be a giant escalation, but even lesser action against Tehran's proxies could fuel the fires of conflict, while destabilizing efforts to reach a ceasefire in Gaza.

Further involvement would undermine Biden's prized policy of extracting America from its "forever wars" in the Middle East -- even if the chaotic US withdrawal from Afghanistan on his watch led to a Taliban takeover.

"There's a domestic political risk, which is Biden alienates part of his base progressives, anti-war folks, and at the same time, opens himself up to accusations of wag-the-dog," said Clarke, referring to a movie in which a US president starts a war to distract from political issues at home.

"So I think it's a really difficult situation."