Smaller than expected interest in the Greek debt buyback that closed on Tuesday means that to reach its debt reduction targets Athens may also offer to repurchase bonds issued before the debt restructuring in March, a euro zone document showed.

The document, dated December 12, said the Greek buyback of debt restructured in March amounted to 31.9 billion euros, which would reduce the Greek debt-to-GDP ratio by 9.5 percentage points by 2020.

When plans for new Greek debt-reduction steps were agreed among euro zone finance ministers on November 27, the expectation was that the buyback would reduce the debt by 11 percent of GDP.

The buyback, along with other steps, is to cut Greek debt to 124 percent of GDP in 2020, from almost 190 percent expected next year without such steps.

The document said the 1.4 percent of GDP reduction in debt - the amount by which the buyback fell short of earlier expectations - could be obtained through a buyback of bonds still held by investors who refused to participate in the Greek debt restructuring earlier this year.

"Euro area member states will consider additional further measures and assistance, if necessary, to bring the total yield of additional contingency measures to 1.4 percent of GDP, provided that Greece meets the 2013 primary balance target as defined in the MoU (memorandum of understanding)," a footnote in the document said.

(Reporting By Matthias Sobolewski, writing by Jan Strupczewski; editing by Rex Merrifield)