FRANKFURT (Reuters) - Buying bonds due to mature further into the future would boost the effectiveness of the European Central Bank's aggressive money-printing program, an ECB research paper said on Friday.

The ECB is looking at options to ensure its the 80 billion-euros-a-month scheme, due to end in March at the earliest, can continue as inflation remains stubbornly low and eligible bonds in countries such as Germany risk becoming hard to find.

The authors of the paper, which does not necessarily reflect the view of the ECB, say that by buying bonds with a longer maturity, the central bank would take on some of the risk currently borne by private investors owning those notes, freeing up resources for lending.

The ECB currently buys sovereign bonds with maturities from two to 30 years and the average maturity of its portfolio is 8.28 years.

The paper estimates that increasing the average maturity of the bonds owned by the ECB to 11 years would increase inflation at its peak by 50 basis points and GDP by 1.4 percent, compared with 40 basis points and 1.1 percent respectively with an average maturity of eight years.

"Purchasing assets with higher duration would eliminate more interest rate risk from the portfolios of financial intermediaries, which would allow them to extend lending," the five ECB economists said in the paper.

The authors also say that giving investors explicit guidance as to future purchases, including the conditions under which they would be extended, and a date when the ECB plans to raise interest rates would also be desirable.

ECB President Mario Draghi unnerved bond markets on Thursday by failing to hint at the anticipated extension of its asset purchases, maintaining instead the March end-date.

The authors of the paper also play down worries that low interest rates spreads are eating into the profit of financial companies, pointing out banks have gained from the bond rally triggered by ECB purchases.

"The results in the paper suggest that this effect was probably more than compensated by the capital relief channel," they said. "Nevertheless, financially stability risks need to be carefully and continuously monitored."

(Reporting by Francesco Canepa; Editing by Alison Williams)