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American Water , the largest water utility in the U.S., disclosed that it had been hit by a cyberattack.

The Camden, New Jersey-based company said in a security statement on its website that it had learned of "unauthorized activity in our computer networks and systems" last Thursday, which it determined "to be the result of a cybersecurity incident."

The company said on Tuesday that it shut down its customer service portal, and as a result, its billing function "until further notice" and will not charge any late fees or other fees related to billing as long as the system is down.

Some recent hacks of major U.S. companies have brought key online systems to a halt and created chaos for consumers and businesses, such as the hack of UnitedHealth which led to nationwide difficulty among patients needs prescriptions filled and health-care professionals needing to be paid for services.

Hacks targeting U.S. water infrastructure, in particular, have been increasing, with some of the attacks linked to geopolitical rivals of the U.S., including Iran, Russia and China.

Taking out critical national infrastructure has become a top priority for foreign-linked cybercriminals. "All drinking water and wastewater systems are at risk — large and small, urban and rural," an EPA spokesman recently told CNBC.

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