The Foreign Secretary was asked about the amount at a Chatham House event on Wednesday, where she set out her foreign policy aims.
Ms Truss said the Government was going to “work night and day to prevent the Iranian regime from ever getting a nuclear weapon”.
But she said they were also working to “resolve the issue” over the debt – relating to a cancelled order for 1,500 Chieftain tanks dating back to the 1970s – which has been linked to the continued detention of Nazanin Zaghari-Ratcliffe and other UK-Iranian dual nationals held in the country.
Ms Truss said: “We do want to pay this debt, we recognise it’s a legitimate debt.
“But of course, there are lots of issues, which I’m sure you are quite well aware of.”
She added that she had spoken to her Iranian counterpart, but said: “It is not simple, for various reasons.”
And she said: “I’m also pressing for the return of our unfairly detained British nationals, including Nazanin.”
Ms Truss also warned Iran that a meeting in Vienna on Thursday was the country’s “last chance” to revive a nuclear deal.
The Foreign Secretary previously said a meeting at the end of November was the country’s last opportunity to agree to the original JCPOA (Joint Comprehensive Plan of Action, known as the Iran nuclear deal).
On Wednesday she said: “This is really the last chance for Iran to sign up, and I strongly urge them to do that, because we are determined to work with our allies to prevent Iran securing nuclear weapons.
“So they do need to sign up to the JCPOA agreement. It’s in their interest.”
Iran has ramped up its uranium enrichment since the US, under former president Donald Trump’s leadership, withdrew from the landmark nuclear agreement between world powers and Iran in 2018.
The US has offered to lift some sanctions in exchange for Iran returning to full compliance with its nuclear commitments.
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in