Trump said the announcement would be made at 7:00 pm (0000 GMT) but did not give more detail.
His comments came after South Korean National Security Advisor Chung Eui-yong visited Washington to brief US allies about recent landmark talks with North Korean leader Kim Jong Un in Pyongyang.
South Korea announced Tuesday that the North had stated there was "no reason" to hold on to its nuclear weapons "if military threats towards the North are cleared and the security of its regime is guaranteed".
President Donald Trump welcomed the offer as "very positive."
However his Director of National Intelligence Dan Coats told Congress he was "quite skeptical" and Vice President Mike Pence said the US position towards North Korea would not change "until we see credible, verifiable, and concrete steps toward denuclearization."