HOUSTON, March 20 (Reuters) - Canada's Trans Mountain

has begun filling its pipeline expansion with oil in a

staged process, a senior executive said on Wednesday, as

construction of the long-delayed progress nears an end.

The pipeline expansion faces technical challenges in the

next two weeks as it finishes work on the last segment in

British Columbia, Chief Financial Officer Mark Maki told

Reuters.

(Reporting by Arathy Somasekhar in Houston and Rod Nickel in

Winnipeg, Manitoba)