Police have launched an investigation after a man's body was found in a Sydney river.

Officers were called to Lane Cove River in West Pymble after the man's body was found in the river about 6.50am today.

"Officers attached to North Shore Police Area Command believe the body is that of a man aged in his 50s," NSW Police said in a statement.

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"A crime scene has been established, and detectives have launched an investigation into the incident."

Police are urging anyone with information to come forward.