In the Saarland state vote held six months before a general election, Merkel's Christian Democrats (CDU) won 40 percent against 30 percent for the Social Democrats (SPD), according to early results reported by public broadcaster ZDF.

Broadcaster ARD estimated a slightly bigger lead for the CDU of 41 percent against 29.5 for the SPD.

The vote was held in the tiny state of Saarland on the French border, which has a population of only one million, but was seen as a bellwether ahead of the September 24 general election in which Merkel is seeking a fourth term.

The SPD have made strong gains in national opinion surveys since Martin Schulz, the former president of the European Parliament, took over in January, and is now polling neck-and-neck nationally with Merkel's CDU.

But the "Schulz effect" did not translate into the strong results the SPD had hoped for in Saarland, a former coal mining region, which held the first of three German state polls scheduled in the run-up to the national election.