The White House has blamed an Iran-backed militia umbrella group for a deadly attack on a US base in Jordan, as it considers how to hit back.
National Security Council spokesman John Kirby said US intelligence believed the Islamic Resistance in Iraq was behind Sunday's drone strike.
President Joe Biden has said without elaborating that he has already decided what the US response will be.
The strike killed three US soldiers and injured at least 40 other US troops.
The "attribution that our intelligence community is comfortable with is that this was done by the umbrella group", said Mr Kirby in a daily press briefing on Wednesday.
The Islamic Resistance in Iraq - which is believed to contain multiple militias that have been armed, funded and trained by Iran's Revolutionary Guards force - has already claimed responsibility for the strike.
The national security spokesman repeated that the US response would be "in a time and in a manner of our choosing, on our schedule".
"Just because you haven't seen anything in the last 48 hours, it doesn't mean that you're not going to see anything," he said.
"The first thing you see won't be the last thing," he added.
Mr Biden has said he does not want a wider conflict in the Middle East, which has already been destabilised by the Israel-Gaza war that erupted in October.
Iran has denied any role in the drone attack.
At an event on Wednesday, Revolutionary Guards commander Major General Hossein Salami, who advises Supreme Leader Ayatollah Ali Khamenei, said that Tehran hears the "threats coming from American officials".
"No threat will be left unanswered," he added.
His remarks came a day after Amir Saeed Iravani, Iran's ambassador to the UN, told Iranian journalists in New York that Tehran would "decisively respond" to any attack on the country, according to the state-run Irna news agency.
While Washington weighs its response, Kataib Hezbollah, part of the Islamic Resistance in Iraq, said on Tuesday it had suspended operations against US forces to avoid causing "embarrassment" to the Iraqi government.
Three American soldiers who were based at Fort Moore, in the US state of Georgia, died in the drone strike in north-eastern Jordan.
At least 41 National Guard members were injured, US officials confirmed on Wednesday.
The injured soldiers are from units based in Arizona, California, Kentucky and New York.
Twenty-seven were able to return to duty while 14 others continue to be medically evaluated, the US National Guard Bureau said.
Meanwhile, the Iran-aligned Houthis in Yemen have continued to carry out missile and drone attacks on merchant ships in the Red Sea in response to the war in Gaza.
A ship in the Gulf of Aden was struck by a missile fired from the Taiz region, according to Ambrey, a maritime security firm.
A spokesman for the Houthis confirmed the attack.
US Central Command, which oversees US military actions in the Middle East, said that the USS Carney shot down one Houthi-fired missile headed towards the Gulf of Aden on Wednesday night.
The US naval destroyer also "engaged and shot down" three Iranian drones "in its vicinity".
Officials also said US fighter jets destroyed 10 drones in Yemen that were preparing to be launched.
According to the Pentagon, the Houthis have launched more than 30 attacks on commercial ships since 19 November.
The ongoing attacks have disrupted global trade and triggered retaliatory strikes from both American and British forces.