The US government sanctioned Lebanon-based Jammal Trust Bank for aiding and transferring money to the Iran-backed group Hezbollah.
Four people providing money for the group and the Palestinian militant organisation Hamas were also sanctioned.
US officials announced the move on Thursday, calling the lender “Hezbollah’s bank of choice” and threatening more action on any entity or people in Lebanon who provide financial help to the militant group.
A senior US official said that “Hezbollah allies could be targeted” if they were found to be in breach of the sanctions.
The Department of Treasury’s Office of Foreign Assets Control, or Ofac, said the bank enabled “banking activities for Hezbollah” and had been added to the executive sanctions order for financially supporting the group.
The order was introduced by former US president George W Bush in 2001 after the September 11 bombings in New York City.
“Jammal Trust provides support and services to Hezbollah’s executive council and the Martyrs Foundation, which funnels money to the families of suicide bombers,” said Sigal Mandelker, Treasury's undersecretary for terrorism and financial intelligence.
“The US will continue to work with the Central Bank of Lebanon to deny Hezbollah access to the international financial system.
"This action is a warning to all who provide services to this terrorist group.
The US Treasury, in partnership with Oman, also sanctioned financial facilitators it says were responsible for moving tens of millions of dollars between Iran’s Islamic Revolutionary Guard Corps Quds Force and Hamas's operational arm, the Ezzedine Al-Qassam Brigades in Gaza.
The four – Muhammad Sarur, Kamal Awad, Fawaz Nasser and Muhammad Al Ayy – are accused of providing financial and technological support to Hamas.
A senior US official told The National that the Treasury did not expect the sanctions to add to the vulnerability of Lebanon's fragile economy.
“This is the 26th largest bank in Lebanon, with a limited role,” the official said.
But Washington hoped that the Lebanese Central Bank would take the appropriate action to implement the sanctions.
The sanctions come two weeks after Lebanon’s Prime Minister, Saad Hariri, visited Washington where he met senior US officials and discussed the issue of Hezbollah.
Mr Hariri acknowledged the limits his government faced with the issue of US sanctions on the party or its allies.
“We can’t change the US will on sanctions," he said then. "They have laws and are pursuing them.
“We are trying to shield Lebanon. We explained our view and we know our message was received.
“Any name or entity that comes on the US Ofac list, we deal with it firmly to shield Lebanon,” Mr Hariri said.
He was referring to Lebanon’s past efforts in absorbing sanctions on the Lebanese Canadian Bank and other people and entities accused of making funds available to Hezbollah.
Over the past few months Washington has increased its pressure on Hezbollah.
It sanctioned security official Wafic Safa and high-ranking parliamentarian Mohamad Raad in July.
It also pledged to do more to squeeze Iran and its proxies across the region.
US Secretary of State Mike Pompeo welcomed the sanctions on Jammal Bank.
"Today’s designation reflects our determination to counter Hezbollah’s terrorist and illicit activities in Lebanon," he said.
Mr Pompeo said the US was "working closely with the Central Bank of Lebanon and other Lebanese institutions, which work to preserve the integrity and stability of Lebanon’s banking system."