VANCOUVER, British Columbia, April 8 (Reuters) - National Bank of Canada (NA.TO: Quote, Profile, Research) announced extra financial assistance on Tuesday for corporate and commercial clients saddled with troubled asset-backed commercial paper.

National, Canada's sixth biggest bank, said it will offer improved credit facilities to about 100 business clients from now until the maturity of notes, which are due to be issued under a restructuring plan for the frozen market.

National agreed last August to buy about C$2 billion ($1.98 billion) in ABCP from its mutual funds and retail clients after the bank had encouraged customers to invest their money in the money market securities.

It did not make the same offer to its business clients although it has made available C$670 million in credit facilities to them. National said on Tuesday that clients had used C$119 million of these facilities as of Feb. 18.

The additional support will see National make credit facilities available to business clients for an initial period of two years after the restructuring proposal is adopted, with the possibility of extending this annually.

Investors in ABCP are due to vote on the proposal on April 25. The plan was hammered out by a private committee of investors and envisages turning the short-term notes into longer-dated investments.

The C$32 billion market for ABCP issued by groups other than Canada's banks stopped functioning last August when buyers dried up on fears the investments were exposed to the U.S. subprime mortgage market.

The credit facilities will cover up to 75 percent of the value of the investment, National said. For clients, the cost of the facilities will mostly be offset by the interest received on long-term notes.

National's offer does not extend to financial institutions, financial intermediaries or government organizations, which it said "can more easily deal with the transformation of ABCP into long-term notes".