Kanye West is reportedly working on a top-secret project with Kid Cudi in Japan

An insider explained: "[It is so secret] they're going to drop some crazy collaboration out of the blue."

Kanye, 40 was hospitalised in November 2016, while Cudi checked himself into rehab last year for "depression and suicidal urges".

But the duo have since returned to the recording studio and are reportedly set to release new material.

The source told the New York Post newspaper's Page Six column: "They're going to drop some surprise project on everybody. They're recording it now."

And the duo have made every effort to keep details of their new project a closely-guarded secret.

The source continued: "Nobody knows what it is - or what it sounds like ... There are no professionals in the room. It's just Cudi, Kanye and an engineer. They're recording together in Japan."

By contrast, Kanye and Cudi became embroiled in a feud last year, when Cudi dissed the outspoken rap star on Twitter after leaving his G.O.O.D. Music record label.

In response, Kanye said during his Saint Pablo tour: "I birthed you ... I'm so hurt. I feel so disrespected. Kid Cudi, we're two black men in a racist world."

But their much-hyped feud seemed to evaporate "pretty quickly", according to an insider.

The source added: "It was likely something that was lingering, but not as serious as it seemed. Kanye's always airing out whatever he's thinking onstage, but they've always been close."

Kanye has largely remained out of the spotlight in 2017, but he was recently seen attending a recording studio with rap group Migos, which prompted speculation he's set to release new material.