The announcements from the Trump administration come amid rising fears about the North's military advances.
The Thaad system was originally not expected to be in use until late 2017. Many South Koreans oppose it, fearing they will become a target.
North Korea has vowed to conduct more missile and nuclear tests.
Speaking to members of the US Congress on Wednesday, Adm Harry Harris, commander of the US Pacific Command, said Thaad would be "operational in the coming days to be able to better defend South Korea against the growing North Korea threat".