United States President Donald Trump is promising "really big news" in a rare primetime address on Thursday night, though he won't say exactly what it is.

The surprise speech was announced on Tuesday. But when pressed by reporters about what he planned to talk about, Trump only revealed that the speech would be about elections and "a couple of other things".

"It doesn't get bigger, because without free and fair elections, you don't have a country," he told journalists in the Oval Office on Tuesday.

Asked to elaborate, Trump said he wanted to "save it" for the speech.

"We'll be discussing other things, too," he added. "It's going to be a very big announcement."

The White House has since confirmed that the address will focus on elections, including information related to the 2020 presidential election, which Trump has falsely claimed he won.

The speech is also expected to discuss what the White House describes as vulnerabilities in US voting machines.

Here's what we know about the upcoming primetime presidential address.

When is Trump's speech?

Trump is expected to speak from the White House on Thursday at 9pm US Eastern Time (01:00 GMT Friday).

How can you watch it?

Major US television networks are expected to carry the address live. The Trump administration has requested airtime from major broadcasters.

It will also be livestreamed on WhiteHouse.gov and on the White House's YouTube page.

Why is the timing significant?

Trump's speech comes three and a half months before the November 3 midterm elections.