Exploratory talks between German Chancellor Angela Merkel's Christian Democratic Union (CDU), its Bavarian sister party the Christian Social Union (CSU) and the center-left Social Democratic Party (SPD) on a possible agreement to start formal coalition negotiations have reached a "breakthrough," sources say.

The agreement on a blueprint for formal negotiations could pave the way to ending months of political uncertainty in German following September elections.

After around 24 hours of talks, the six party and parliamentary leaders began presenting the 28-page blueprint to party members, party sources said.

Merkel had said Thursday the talks would be "tough" but that she hoped the parties would conclude the preliminary round by Friday.

 Read more: Germany's never-ending coalition talks break record

Media blackout

Thirty-nine negotiators (13 from each party) began exploratory talks on Sunday with the goal of hashing out policy agreements that would allow the parties to start detailed coalition negotiations.

So far, leaders have reportedly agreed to drop Germany's target of reducing carbon emissions by 40 percent by 2020.

But few details are known about how much progress the parties have made on other issues amid a secretive negotiating atmosphere.

Read more: Opinion: German coalition hopefuls drop climate goals

Government before Easter?

Many observers and members from all three parties hoped a breakthrough on other issues could be achieved to avoid a CDU/CSU minority government or fresh parliamentary elections.

The final hurdle is set to be the SPD party conference on January 21 in Bonn, where party members are to vote on whether enough progress has been made to start formal coalition negotiations.

Formal talks are not expected to end in a final coalition government before Easter.

Read more: Germany's CSU returns to far-right political battleground

Latest attempt

The latest negotiations come more than three months after national elections saw large losses for the CDU, CSU, and SPD that together had been governing as a "grand coalition" since 2013.

The SPD initially ruled out renewing the coalition, leading the CDU and CSU to seek a coalition with the business-friendly Free Democratic Party (FDP) and the Greens. But exploratory talks fell apart in November after FDP leader Christian Lindner walked out.

Read more: German coalition talks collapse: 'A terrible end is better than unending terror'

amp/rc (dpa, Reuters)