German Chancellor Angela Merkel's conservative party suffered heavy losses in two key regional elections Sunday, early estimates showed, as voters vented anger over pandemic setbacks and a face-mask procurement scandal.

The votes in the southwestern states of Baden-Wuerttemberg and Rhineland-Palatinate were being closely watched as a barometer of the national mood ahead of a general election on September 26 -- when Merkel's successor will be chosen.

In wealthy Baden-Wuerttemberg, Merkel's centre-right CDU was set for its worst-ever result at 23 percent, according to exit polls by public broadcasters ARD and ZDF.

As in the 2016 vote, the Green party took first place again, garnering more than 31 percent.

Baden-Wuerttemberg is Germany's only state run by a Green premier, Winfried Kretschmann, who has been in office since 2011.

He could now choose to maintain his current coalition government with the CDU, or build a new one with the centre-left SPD and the pro-business FDP, which each took around 10 percent of votes.

In neighbouring Rhineland-Palatinate, the CDU placed second with 25-26 percent of votes, down from almost 32 percent in the previous regional election.

The centre-left SPD shed some support but held onto first place, at 33-34 percent, according to the estimates.

The result paves the way for popular SPD state premier Malu Dreyer to continue governing with the pro-business FDP and the Greens, who more than doubled their score.

Because of the pandemic, a higher than usual number of votes were cast by mail, and observers cautioned that the final results could still change as ballots continued to be counted.

If confirmed, the results mark a worrying start for the CDU/CSU to what has been dubbed Germany's "super election year".

Merkel's federal government, which includes the SPD as junior partner, initially won praise at home and abroad for suppressing the first coronavirus wave last spring.

But it has increasingly come under fire over Germany's sluggish vaccination campaign, a delayed start to free rapid testing, and a resurgence in cases despite months of shutdown.

The CDU and its Bavarian CSU sister party have also been roiled by damaging claims about MPs apparently benefitting financially from face mask deals early on in the pandemic, forcing three lawmakers to step down in recent days.

The mask scandal "weighed heavily on the election fight", said CDU secretary general Paul Ziemiak.