Rapper Eminem has slammed US presidential candidate Donald Trump as a “loose cannon” in a new track called Campaign Speech.

He dropped the nearly eight-minute-long, politically charged song, which also mentions fatally shot teenager Trayvon Martin, to announce upcoming new music.

The 44-year-old — real name Marshall Mathers — wrote on his social media pages: “Don’t worry I’m working on an album! Here’s something meanwhile.”

Lyrics in Campaign Speech include an angry attack on Republican candidate Mr Trump.

“Consider me a dangerous man, but you should be afraid of this dang candidate,” he raps.

“You say Trump don’t kiss ass like a puppet? ’cause he runs his campaign with his own cash for the funding?

“And that’s what you wanted? A f***in’ loose cannon who’s blunt with his hand on the button, who doesn’t have to answer to no one? Great idea!”

Another assault comes from Eminem towards the supporters of Mr Trump as he raps: “Run the faucet, I’m a dunk a bunch of Trump supporters underwater. Em flows at one point over minimalist swabs of synth. Snuck up on ’em in Ray-Bans in a grey van with a spray tan.”

Part of the lengthy and musically minimal track references George Zimmerman, the former neighbourhood-watch volunteer who shot dead unarmed black teenager Martin in a 2012 confrontation.

Eminem’s last record was The Marshall Mathers LP 2 in 2013, his eighth studio album.

The hip-hop performer, whose hits have included Stan and My Name Is, has yet to reveal more details of his forthcoming effort.