Extended CO emission in the field of the light echo of V838 Mon

V838 Mon erupted at the beginning of 2002 becoming an extremely luminous star with L=10^6 L_sun. The outburst was followed by the spectacular light echo that revealed that the star is immersed in a diffuse and dusty medium, plausibly interstellar in nature. Low angular resolution observations in the lowest CO rotational transitions revealed a molecular emission from the direction of V838 Mon. The origin of this CO emission has not been established. In this paper we investigate the idea that the molecular emission originates in the material responsible for the optical light echo. We report on observations of 13 positions within the light echo in the two lowest rotational transitions of CO using the IRAM 30 m telescope. Emission in CO J=1-0 and J=2-1 was detected in three positions. In three other positions only weak J=1-0 lines were found. We conclude that the molecular emission from the direction of V838 Mon is extended and has a complex distribution. We identify the emission as arising from diffuse interstellar clouds and suggest that the CO-bearing gas and the echoing dust are collocated in the same interstellar cloud.