B2K's Raz B and Wife Mona Li's Infant Son Astro Dead at 3 Months Old

B2K singer Raz B and his wife, Mona Li, announced the devastating loss of their baby son Astro Monte Thornton ... just three months after he was born.

Sources close to the family tell TMZ ... Astro died earlier this week, and his death was unexpected and sudden.

The cause of the infant's death is unclear.

A family spokesperson tells us ... "It is with heavy hearts that we announce the unexpected passing of Astro Monte Thornton, beloved son of Mona and De'Mario 'Raz B' Thornton of the R&B supergroup B2K. The family asks for privacy as they navigate this difficult time. They truly appreciate your love and support."

Raz announced the soon-to-be arrival of Astro on May 18, writing on Instagram ... "Right now we're at the hospital getting ready to welcome our newest addition, Astro ... another baby boy to the Thornton family."

The couple -- who also share son Versai Carter Thornton -- also shared a sweet post from Versai's Instagram account, which they manage, on May 28, writing ... "Best week ever ... We welcomed my new baby brother Astro into the family, and it already feels amazing being a big brother."

Raz recently celebrated Father's Day with the family, sharing on their joint Instagram ... "Thank you for your love, commitment and generosity for us and those you care for."

In the accompanying video, Raz B could be seen sweetly holding his infant son.