MIAMI, Oct 14 (Reuters) - Puerto Rico, the Virgin Islands, and a slew of other Caribbean islands were put under a hurricane watch on Tuesday as Tropical Storm Omar strengthened, the U.S. National Hurricane Center said.

The storm's sustained winds grew to 70 miles per hour (113 km per hour), just under the threshold for a hurricane, as it churned about 345 miles (550 km) south-southwest of San Juan, Puerto Rico, the Miami-based hurricane center said. (Reporting by Jim Loney, Editing by Michael Christie)

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