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COLOGNE, Germany (Reuters) - German Chancellor Angela Merkel urged private housing investors on Friday to recognize their obligation to the public amid growing concern that spiraling rents in Germany’s biggest cities are pricing out locals and choking off economic growth.

Speaking at a nationwide congress of German tenants, Merkel said “public-spirited” private investors had a crucial role to play in solving Germany’s housing crisis, and that authorities should make sure to use all tools at their disposal to regulate landlords.

Thousands took to the streets of the German capital in April demanding the expropriation of more than 200,000 apartments sold off to big private landlords. Much of their ire was directed at companies like Deutsche Wohnen, Berlin’s biggest landlord with more than 115,000 flats, and Vonovia.

“We need private investors who must also feel a sense of public spiritedness,” she said. “There are questions over whether they have always have,” she said without naming specific investors. “I’m not that happy about every development.”

Building homes was the best answer to housing shortages, she said.

“It must remain attractive for a group of people who feel a sense of obligation to this country to invest in housing.”

The issue is particularly sensitive in Germany, where rents have traditionally been relatively stable over many decades, leading to a culture where middle-class families live in rented homes throughout their lives.

Merkel did not address a proposal from Berlin’s housing senator for a five-year rent freeze for all existing housing.

Activists in the capital announced on Friday that they had collected almost 80,000 signatures for their campaign to demand a city-wide referendum on expropriating the landlords.