"We have split up – suddenly and recently," the Russian musician and singer confirmed at a press conference in Moscow, where she is visiting for a charity event. "Unfortunately, I cannot give you the reason. But you will find out everything quite soon."
She added: "Here is the official version: We split up by mutual consent, and we will raise our daughter together. She is currently with my mother in my Los Angeles home."
Grigorieva and Gibson welcomed a daughter, Lucia – the 54-year-old actor and director's eighth child – last October.
A source tells PEOPLE that Grigorieva and Gibson had "issues they couldn't solve." One issue was apparently how much time they spent together. "Mel likes his freedom, while Oksana felt they should spend every minute together," a source says of the pair.
At the Moscow press conference, Grigorieva did thank Gibson for his help with her music career. "Mel himself has directed several video clips for my songs," she said. "He has written lyrics for two of my songs and has always greatly supported me. He also gave me an incredible opportunity to become a producer of my own album. It doesn't happen a lot these days."