By Andrea Shalal

BERLIN, Jan 23 (Reuters) - A leading member of Chancellor Angela Merkel's conservatives has urged Social Democrats (SPD) to finish coalition talks within two to three weeks, warning that Germans were losing trust in democracy after months without a new government.

Volker Kauder, who leads conservatives in parliament, rejected calls by Martin Schulz, the embattled leader of the centre-left 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" discusions 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.

DROPPING SUPPORT FOR SPD

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).