Taiwan raised its warning level for travel to China on Thursday, urging its citizens not to visit there unless necessary after Beijing spelled out potential punishments — including execution in extreme cases — for what it called “diehard supporters of Taiwanese independence.”

       China regards Taiwan, a democratically governed island of 23 million about 100 miles off the mainland coast, as its territory. It demands that Taiwan ultimately accept unification and has long denounced Taiwanese who oppose its claims to the island.

       Last week, China turned up the pressure, issuing legal guidelines that detailed the steps it might take to punish supporters of Taiwanese self-rule. They came as tensions between U.S.-backed Taiwan and China ratchet ever higher. Last month, Taiwan swore in a new president, Lai Ching-te, who has vowed to preserve democracy on the island and is denounced by Beijing.

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