Berlin (AFP) - Germany's powerful car industry offered Wednesday to provide a software upgrade that would cut harmful emissions in 5.3 million vehicles, but the government warned it was only a first step in dredging the sector out of a colossal pollution cheating scandal.

In a bid to head off threats by major cities to partially ban diesel vehicles in their fight against deadly smog, bosses of auto groups Volkswagen, Daimler, BMW and Opel promised a patch that would reduce nitrogen oxide (NOx) emissions by between 25 and 30 percent for cars that have been on the roads for less than nine years.

The carmakers will also stump up incentives for drivers to trade in cars that have been in circulation for ten years and more, said ministers after a meeting with auto industry executives.

Transport Minister Alexander Dobrindt called the measures a "sensible basis" to slash emissions quickly, but Environment Minister Barbara Hendricks said it was only "the first important step", warning that there may still be "gaps that need to be closed".

But at least half of the 5.3 million vehicles were already offered a fix after a pollution cheating scam that engulfed Volkswagen broke in September 2015 -- a point that Greenpeace was quick to point to.

"Instead of protecting millions of people from diesel exhaust fumes, the government is today hooking a dying engine onto an oxygen tank," it charged.

Greg Archer of lobby group Transport and Environment, called the software upgrade a "welcome stop-gap measure" but stressed that "this is not a long-term solution to the air pollution crisis.

"Nor will it make diesel clean," he warned.

- 'Majority back diesel ban' -

Germany's auto sector has been scrambling to rescue its reputation and save diesel motors over the last two years.

The first cracks in the oft-vaunted sector emerged in September 2015, when Volkswagen admitted installing illegal devices in millions of vehicles world-wide to rig pollution emissions readings.

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But suspicions of similar cheating have since widened to other German carmakers, including Mercedes-Benz maker Daimler and BMW.

In July, Spiegel magazine heaped on further pressure as it published details of a VW letter to German and European competition authorities which it said showed that auto giants colluded on technology, suppliers, costs, sales and markets since the 1990s.

Adding to the clouds hanging over the industry, a court in Stuttgart -- the home city of Mercedes and Porsche -- ruled that only a partial ban on diesel vehicles would be effective at clearing the air of poisonous NOx emissions.

Germany has already been warned by the European Commission over its air quality, and now public opinion is starting to swing in favour of outlawing diesel.

A survey commissioned by Greenpeace found that 57 percent of Germans back such a ban in cities with poor air quality.

Two months before a general election, the issue is shaping up to be a hot potato topic as Chancellor Angela Merkel needs to weigh the interests of a sector with more than 800,000 jobs in the balance and those of millions of car owners against public health.

"The car industry is of strategic importance ... it must be strong and innovative but also honest. So it's about criticising what needs to be criticised, but to do so while bearing in mind that it's a strategically important industry in Germany," said her spokeswoman Ulrike Demmer on Monday.

- 'Don't demonise diesel' -

The problem is that the industry had plumped for diesel as it spews out less climate-altering carbon dioxide than petrol-burning motors.

While electric cars are viewed as green, electricity generation in Germany is not always clean -- with some 40 percent still stemming from coal.

For these reasons, Merkel herself has warned against "demonising diesel".

But the flip side of the technology is that it emits more NOx, contributing to the formation of smog and fine particles that cause respiratory and cardiac problems.

For now, Germany's auto giants are hoping their voluntarily efforts towards lowering emissions will buy them time as they ramp up electric car development.

Some 15 million diesel vehicles are in circulation in Germany. Such cars also made up 40.5 percent of vehicles sold in July -- after suffering a steep fall in popularity.

For Lower Saxony state premier Stephan Weil, "Germany must remain a car country, but please, with cleaner cars."