A top Democratic senator on Thursday questioned whether US President Donald Trump interfered with a criminal investigation into Turkey's Halkbank, which has been accused of conspiring to evade sanctions against Iran.
Oregon Senator Ron Wyden, ranking member on the Senate finance committee, wrote to US Treasury Secretary Steven Mnuchin.
Mr Wyden asked if Mr Trump or anyone acting on the president's behalf had asked him to "handle, intervene or otherwise engage with Turkish concerns related to Halkbank, or with Halkbank generally?"
The letter quoted an October 16 Bloomberg report, in which Mr Trump told Turkish President Recep Tayyip Erdogan in April that Mr Mnuchin and US Attorney General William Barr would handle the Halkbank issue.
Mr Wyden said he expected an answer by November 20.
US federal prosecutors in Manhattan this month charged Turkey's second-largest state bank with fraud and money laundering, other sanctions-related offences, and using sham food and gold transactions to get around US sanctions.
Halkbank has denied the charges.
The US court case has further strained tensions between Turkey and America, which have been at odds over issues including Ankara's Syria incursion and purchase of a Russian missile defence system.
The charges against Halkbank are the latest development in a US criminal case that became public in 2016 with the arrest in Miami of Reza Zarrab, a Turkish-Iranian gold trader accused of playing a central role in a sanctions evasion scheme.
Mehmet Hakan Atilla, a Halkbank deputy general manager, was arrested in New York the following year. He was sentenced to 32 months in prison after his conviction and was released and returned to Turkey this year.
Mr Zarrab pleaded guilty and testified for US prosecutors at Atilla's trial.
Before pleading guilty he hired Rudy Giuliani, a longtime associate of the president, to try to negotiate a deal between the US and Turkish governments to secure his release.
Mr Zarrab said that Iran, with the help of Halkbank and Turkish government officials including Mr Erdogan, used a complex web of shell companies and sham transactions in gold, food and medicine to get around US sanctions.
Turkish officials raised Halkbank in their discussions with Americans this week, US Vice President Mike Pence said in Ankara after the talks.
Mr Pence said the Turks were told this was a matter for the Southern District of New York, whose prosecutors brought the criminal case.
Mr Erdogan has blasted the US charges against Halkbank and called the case unlawful and ugly.