BERLIN (Reuters) - Germany's centre-left Social Democrats (SPD) have overtaken the Greens and are closing the gap with Chancellor Angela Merkel's conservatives less than six weeks before a federal election, an opinion poll showed on Wednesday.

       Support for the conservative bloc led by Armin Laschet, who has been widely seen as frontrunner to succeed Merkel as chancellor after the Sept. 26 election, has ebbed since the end of June, when it was polling at 28%-30%.

       The latest poll by research institute Forsa put the conservatives on 23%, unchanged from a week ago. The Social Democrats (SPD), whose chancellor candidate Olaf Scholz tops the popularity ratings, was up 2 points at 21%.

       This is the slimmest gap between the two parties since March 2017 and opens various coalition options. The most likely scenario until a few weeks ago, a conservative-Greens alliance, now seems less likely.

       The Greens, led by chancellor candidate Annalena Baerbock, were down 1 point at 19%.

       The Taliban takeover of Afghanistan has had little impact on voting intentions although Germany had the second-largest military contingent there after the United States and is trying to evacuate people.

       After 16 years leading Europe's biggest economy, Merkel is stepping down as chancellor after the election. Her bloc, made up of her Christian Democrats and the Bavarian Christian Social Union, looks to be missing her electoral pull.

       The ratings of Laschet, premier of the most populous German state, North Rhine-Westphalia, have dropped since he was seen laughing on a visit to a flood-stricken town and had to apologise.

       Voters widely see Scholz, finance minister in Merkel's coalition, as a safe pair of hands. Some 29% of respondents told Forsa they would back Scholz if there were a direct vote for chancellor, about 17 points ahead of Laschet. Baerbock was on 15%.

       (Reporting by Madeline Chambers, Editing by Timothy Heritage)