Hariri last week said in a TV broadcast from Riyadh he was stepping down because of an unspecified threat to his life. He also attacked Hezbollah and Iran.
Iran and its Lebanese ally, the Shia group Hezbollah, claim that Saudi Arabia has detained Hariri and forced his resignation.
Praising Hariri, the White House said he has been a "trusted partner" of the US in strengthening Lebanese institutions, fighting terrorism, and protecting refugees.
"In this sensitive time, the United States also rejects any efforts by militias within Lebanon or by any foreign forces to threaten Lebanon's stability, undermine Lebanese government institutions, or use Lebanon as a base from which to threaten others in the region," the White House said in a statement.
The US firmly reiterates that the Lebanese Armed Forces and other Lebanese state security forces are the only legitimate security authorities in Lebanon, the statement said.