Iran's president last week said the U.S. was ready to lift trade sanctions, although a senior Iranian official contradicted him and European diplomats said difficult issues remained.
Indirect talks have been underway in Vienna as the Biden administration seeks a path forward with Iran, including how Tehran can resume compliance with its 2015 nuclear deal with world powers.
"Iran, I think, knows what it needs to do to come back into compliance on the nuclear side, and what we haven't yet seen is whether Iran is ready and willing to make a decision to do what it has to do. That's the test and we don't yet have an answer," Blinken told ABC News' "This Week With George Stephanopoulos" program.
With a fifth round of talks in the coming days, Blinken said the discussions have clarified what both sides needed to do. "We've actually made progress," he told CNN's "Fareed Zakaria GPS" program.
The unilaterally U.S. abandoned the international pact in 2018 under former Republican President Donald Trump, who reimposed sanctions on Iran's oil, banking and shipping sectors.
Democratic President Joe Biden, who succeeded Trump in January, has said he believes Iran is seriously engaging in talks but that it remained unclear what steps Tehran would actually take to comply with the deal, which was cemented when Biden was vice president under former President Barack Obama.
If both sides can return to the original deal, Blinken told CNN Sunday, "then we can use that as a foundation both to look at how to make the deal itself potentially longer and stronger – and also engage on these other issues, whether it’s Iran’s support for terrorism ... its destabilizing support for different proxies throughout the Middle East."
But, he said on ABC, "The first thing that we need to do is put the nuclear problem back in the box."
