Doctors believe that Gibb, 62, has a secondary tumor, his representative Doug Wright said, confirming a news account in The Sun in the United Kingdom. Gibb had emergency surgery in 2010 for a blocked bowel and then had more surgery for a twisted bowel, Wright confirmed.
"We are all hoping and praying that he will pull through," said the message on Gibb's website, which noted that reports "are true" about Gibb being in a coma and having contracted pneumonia.
News of Gibb's condition elicited a flood of support on social network sites and in the music community.
Peter Frampton, who like the Bee Gees rose to fame in the 1970s, wrote on Twitter, "My positive thoughts go out to Robin Gibb and all of the Gibb family. I am wishing you the best and hope for a quick recovery. Much love 2 U."
Others offering their thoughts and prayers online included Julian Lennon, the son of John Lennon, and former Jackson 5 member Jackie Jackson.
Diane Warren -- who herself has written and produced dozens of popular songs for a wide range of performers, including Aretha Franklin and Justin Bieber -- described Gibb as "one of the best songwriters of all time."
Robin Gibb's twin brother, Maurice, died in 2003 of a twisted bowel.
Brother Andy died at age 30 from a heart infection.
The Brothers Gibb -- calling themselves the Bee Gees -- soared to renown as one of the most successful British groups after the 1977 film "Saturday Night Fever" starring John Travolta was built around the group's disco songs.
Inducted into the Rock and Roll Hall of Fame in 1997, the Bee Gees have sold more than 200 million albums, and their soundtrack album to "Saturday Night Fever" was the top-selling album until Michael Jackson's "Thriller" claimed that distinction in the 1980s.
While often in the background as brother Barry sang lead vocals, Robin Gibb stepped forward on several top tunes, including "I Started a Joke" and "I've Gotta Get a Message to You." He also recorded several solo albums during his career.
