Greek socialist party PASOK leader Evangelos Venizelos became on Thursday the third party chief in four days to launch a bid to form a coalition administration, after the May 6 national elections gave no party a clear parliamentary majority.

"Things are not easy. I will not say I am optimistic, but I assume the responsibility focused on this goal, which I believe serves the national interests," the former Finance Minister said after formally receiving a three-day mandate from President Karolos Papoulias.

The conservative New Democracy party chief Antonis Samaras and Leftist SYRIZA coalition head Alexis Tsipras, whose parties ranked the first and second in the Sunday polls, have both returned the mandate to Papoulias after faling to form coalition governments.

If Venizelos fails as well, the Greek president would have to call new elections, a prospect that has fuelled fears for the danger of financial turmoil over missed bailout timetables and targets.

Venizelos is due to begin later on Thursday evening a new round of separate talks with possible coalition partners, starting from Fotis Kouvelis, party chief of the Democratic Left.

Deliberations so far amongst pro-bailout PASOK and New Democracy parties, and anti-austerity Left and Right parties have shown that there were still different views regarding the economic policy Greece should pursue in the future.

PASOK and New Democracy, which have practically ruled the debt-ridden country's political sphere since 1975 but recorded record low support rates in Sunday's elections thanks to unpopular harsh austerity imposed in exchange of international bailout packages since 2010, warned that any major changes in the commitments undertaken could end the flow of financial aid deemed vital for Athens to avoid a default.

Venizelos repeated on Thursday that he will try to form a national unity administration with a clear pro-European agenda. But he said he could seek a review of some terms of the contracts signed with EU and International Monetary Fund creditors, in order to ease the pressure on Greek recession-hit citizens.