US President Donald Trump will meet North Korean leader Kim Jong-un in Singapore on 12 June.
The pair had previously exchanged insults and threats. A breakthrough came after landmark talks between North and South Korea.
No sitting US president has ever met a North Korean leader. The White House said the release of three Americans was a gesture of goodwill ahead of the summit, which Mr Trump earlier said he thought would be a “big success”.
“I really think we have a very good chance of doing something very meaningful,” he said.
The key issue expected to be discussed is North Korea’s nuclear weapons programme – over which Mr Trump and Mr Kim furiously sparred in 2017. The US wants Pyongyang to give up its weapons programme completely and irreversibly.
But analysts caution that Mr Kim is unlikely to easily abandon nuclear weapons that he has pushed so hard to obtain, and that “denuclearisation” means something quite different to both sides.