The US and Iran have agreed to a prisoner swap involving five Americans and five Iranians. US President Joe Biden also unlocked Iranian oil revenue worth nearly 6 billion dollars that had been frozen in South Korea.

The Americans were released on Monday. They were sent to Qatar for a health check, then boarded a plane for Washington. They include businesspeople, an environmentalist, and a worker for the United Nations.

Iranian authorities jailed them on charges of spying. US officials have claimed that they are innocent.

Biden agreed to free up oil funds that were frozen after former President Donald Trump pulled the US out of a nuclear deal with Iran in 2018. Trump accused the Iranians of using their nuclear program to develop weapons.

Officials in the Biden administration say they will make sure the money is spent only on food, medicine, and other humanitarian goods. Republicans, however, are accusing them of helping to finance terrorist activities.

Secretary of State Antony Blinken said the US will take "whatever step is necessary" to deal with actions by Iran in a "whole host of areas that are profoundly objectionable."

Blinken added that the prisoner swap does not represent a new phase of relations with Iran but stressed that diplomacy is the best way to get a "sustainable, effective result" in containing Tehran's nuclear program.