FRANKFURT (Reuters) - The European Central Bank cannot yet specify a timeline to include Greece in its 80-billion-euro ($89-billion) a month asset purchase program, ECB President Mario Draghi said in a letter to European lawmaker published on Wednesday.

Greece needs to pass an ECB debt sustainability analysis before it can be included but measures to cut its debt pile are still under consideration, making the analysis difficult.

"A precise timeline for the possible purchase of marketable instruments issued or guaranteed by the Hellenic Republic under the PSPP (Public Sector Purchase Programme) cannot be specified at the current juncture," Draghi said.

(Reporting by Balazs Koranyi; Editing by Louise Ireland)