Volker Kauder, who leads Chancellor Angela Merkel's conservatives in parliament, rejected calls by Martin Schulz, the embattled leader of the center-left Social Democrats (SPD), for revisiting key issues already agreed in a coalition blueprint.
Schulz met with Merkel and the leader of the Bavarian CSU conservatives, Horst Seehofer, on Monday evening for what party sources called "constructive" discussions about the way forward on building a new government.
The political blocs are due to meet separately on Tuesday before formal coalition negotiations, which could start later this week. Failure to reach agreement on a coalition could result in a minority government or new elections.
"We should finish the coalition negotiations in two to three weeks," Kauder told the Funke Mediengruppe newspaper chain in an interview published on Tuesday. "The citizens are tired of waiting. Every day that passes without a new government does not exactly increase trust in the parties and democracy."
A weary Schulz, whose leadership was in play on Sunday, told reporters on Monday that he aimed to build a government that "improved the lives of the people in this country, but also meets Germany's international commitments, especially with regard to the (European Union) and the unity of Europe."
Earlier, he said negotiators "will talk about all the topics we addressed in the exploratory talks again".
Conservatives reject a wholesale re-look at the blueprint, arguing that it would delay and complicate the negotiations.
"We shouldn't even talk about improvements. The blueprint is the basis for a coalition," Kauder told the Funke group.
Only 56 percent of SPD delegates at party congress on Sunday voted to launch formal coalition on the basis of that blueprint.
The narrow victory margin - and continued opposition by the party's youth wing - puts pressure on Schulz to secure further concessions from conservatives on immigration and healthcare, something that conservatives have rejected thus far.
The SPD had wanted to go into opposition instead of redoing the tie-up that ruled Germany the past four years after gaining just 20.5 percent of the vote in September national elections, the worst result for Germany's oldest party since 1933.
A new RTL poll conducted on Monday showed the party's support had dropped a point to 17 percent, just four points ahead of the far-right Alternative for Germany (AfD).
The stakes are high because Schulz has promised that the SPD's 443,000 members will be allowed to vote on a final deal.
For Merkel, a rerun of the conservative-SPD Grand Coalition that has governed Europe's economic powerhouse since 2013, is her best shot at securing a fourth term as chancellor.
She said she looked forward to intensive talks on forming a stable government and her priorities were preserving Germany's economic strength and ensuring social justice and security.
Investors and partner countries are worried that policymaking in Germany and Europe may become hamstrung by a political deadlock that is about to enter its fifth month.
Wolfgang Tiefensee, designated leader of the SPD in the eastern state of Thuringia, on Tuesday urged Schulz to stick to his vow to refrain from seeking a ministerial post in a new government.
"An 180-degree pivot on this issue would shatter the credibility of Martin Schulz," Tiefensee, a former federal transportation minister, told Die Welt newspaper.
