(Reuters) -- Greek conservative party leader Antonis Samaras has not yet signed a written commitment to the country's international lenders on implementing an austerity package, a source close to negotiations on securing a new EU/IMF bailout said on Tuesday.

So far Samaras has not given a letter of commitment and this is a problem, the source told Reuters on condition of anonymity.

His New Democracy party declined comment.

A separate source at the PASOK socialist party said its leader, George Papandreou, had already provided a signed undertaking to support the measures passed by parliament early on Monday.

(Reporting by Renee Maltezou; editing by David Stamp)