The Diffie-Hellman algorithm is used for Key agreement (key distribution) and cannot be used to encrypt and decrypt messages. Source: WALLHOFF, John, CBK#5 Cryptography (CISSP Study Guide), April 2002 (page 4).
Note: key agreement, is different from key exchange, the functionality used by the other asymmetric algorithms.
References: AIO, third edition Cryptography (Page 632) AIO, fourth edition Cryptography (Page 709)