Maurizio Martina, interim leader of the PD, said although "important progress had been made" during the latest round of consultations, he could not "hide the difficulties" which lay ahead.
"We are different political forces with very different points of view," he said of a potential partnership with the M5S, adding that a decision was postponed until May 3rd.
Italy has been gripped by a political stalemate since the March 4th election failed to produce a clear winner.
A first round of consultations between M5S leader Luigi Di Maio and the far-right League's Matteo Salvini collapsed last week after both refused to budge over Salvini's coalition partner, former premier Silvio Berlusconi.
Di Maio had demanded that Salvini dump the 81-year-old media magnate, who the M5S regards as the symbol of political corruption, but Salvini insisted he would not break up a coalition that came first in the elections with a combined 37 percent. The M5S is Italy's largest single party with 33 percent.
The political impasse led Di Maio to slam the door on any future accord with the League and turn towards the centre-left Democratic Party in a fresh round of negotiations.
The PD, in power since 2013, only got 23 percent of the vote as part of a left-wing coalition. It had until now, insisted it would remain in opposition, refusing to be a "crutch" for a M5S administration.
But this week Martina said his party was willing to come to the table and negotiate.
"If we have got to this point, it is because the others have failed," he said. "We are doing this work with the spirit of duty that we have always guaranteed the country."
Any accord between the two parties is likely to ruffle feathers given that the M5S targeted the PD with ferocious criticism when it governed Italy. Their majority would also only be razor thin and the significant policy differences between the two would make forming a government no easy feat.
After the round of talks concluded, the speaker of the lower house Roberto Fico said: "Talks between M5S and the PD have been launched and in the coming days there will be discussions."
M5S leader Di Maio said he is ready to sign an "ambitious government agreement" with the PD. He also again insisted that should this attempt to form a government fail, the country should return to the polls.
"If we vote again, I'm convinced that the Five Star Movement will emerge even stronger," Di Maio said.