One of the largest U.S. apartment landlords has filed civil lawsuits in California and Florida against short-term rental website Airbnb Inc., alleging its tenants are renting out units on the site in violation of their leases and are creating unsafe conditions for other tenants.

Apartment Investment & Management Co., or AIMCO, which owns or manages roughly 50,000 apartments across the U.S., is seeking an undetermined amount of damages, a spokeswoman said.

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