Talks over restructuring part of Greece's massive public debt ran into trouble on Tuesday as one fund walked away from negotiations, fuelling growing doubts about whether a deal that is crucial to a new bailout agreement can be reached this year.

Vega Asset Management, a Madrid-based fund, resigned from the steering committee representing private creditors negotiating a voluntary restructuring of Greek government bonds, sources familiar with the situation said.

The fund has threatened legal action against officials negotiating the countryâ??s debt restructuring if losses are too deep.