Turkish telecom companies are intercepting traffic sent to public Internet address books run by Google Inc. and other U.S. firms, closing a major loophole Turkish people have used to circumvent a government blackout on social media, the U.S. companies said.

In recent days, Turkish telecoms across the country have started intercepting and redirecting user requests sent to public address books—known as domain-name system servers—run by Google, Level 3 Communications Inc. and OpenDNS, the three companies said Monday.

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