
But the US Treasury said more action, if it had international support, could further isolate the institution.
The state department has said that the US has had "direct contact" with the Iranian government about the allegations.
"We have had direct contact with Iran on this issue," Victoria Nuland, the state department spokeswoman, told a news briefing on Thursday, declining to give further details.
"We are not prepared at the moment to go any further on the question of who spoke to whom, and where, but just to confirm that we have had direct contact with Iran."
'Spreading Iranophobia'
Ayatollah Ali Khamenei, Iran's supreme leader, said on Thursday that the West was trying but failing to instill "Iranophobia", in remarks that appeared to be prompted by, but did not directly address, the US allegations.
Saudi Arabia said it was weighing its response to the alleged plot that has increased tensions between OPEC's two top oil producers.
"We hold them [Iran] accountable for any action they take against us," Prince Saud al-Faisal, the Saudi foreign minister, said in Vienna.
"Any action they take against us will have a measured response from Saudi Arabia."
The White House maintains Iranian plotters hired a would-be assassin in Mexico.
Two men, including one who the US says was a member of Iran's Quds Force special foreign actions unit, were charged in a New York federal court on Wednesday with conspiring to kill the Saudi ambassador to the US, Abel al-Jubeir, at a Washington restaurant.