MANILA, Jan 13 (Reuters) - The Philippine affiliate of Philip Morris International Inc said on Tuesday it would cut 640 jobs, or nearly 13 percent of its workforce in the country, as it restructures cigarette manufacturing operations at its Marikina plant in Manila.

The plant, which employs 1,640 workers in all, will see its workforce reduced to 1,000 once the job cuts take effect on Feb. 15, said a company source who did not wish to be identified.

The Philippine affiliate PMFTC Inc, a joint venture between Philip Morris and unlisted Fortune Tobacco Corp of Philippine tycoon Lucio Tan, employed around 5,000 people in the country prior to the proposed layoffs, with the remaining workers located at another plant in Tanauan City in Batangas province, south of the capital Manila, and at regional sales offices.

"This was a very difficult decision to make but in order to maintain a viable operation and to safeguard the future of our business in Marikina, it was necessary to take this step," PMFTC Inc President Paul Riley said in a statement.

Production volume at the Marikina plant had declined by over 30 percent since 2012 as a result of illicit cigarette trading and higher tobacco taxes that were imposed from January 2013, Riley said.

The affected employees would be given "very generous" separation packages, well in excess of the legal requirements, Riley said.

He said PMFTC would "continue to offer highly competitive salary and conditions for all our employees, as a committed long-term investor, employer and major taxpayer in the Philippines."

(Reporting by Neil Jerome Morales; Editing by Biju Dwarakanath)