MIAMI (Reuters) - Tropical Storm Chris, the third of the Atlantic hurricane season, formed on Tuesday in the Atlantic Ocean east of the Leeward Islands, prompting storm alerts for some of the small northeastern Caribbean islands and Puerto Rico.

Chris, with maximum sustained winds of 40 mph (64 kph), was about 135 miles east of Antigua at 8 a.m. EDT (1200 GMT), the U.S. National Hurricane Center in Miami said.

The hurricane center said Chris was moving to the west-northwest at about 9 mph (14 kph) on a path that would take it north of Puerto Rico and the island of Hispaniola, which is shared by Haiti and the Dominican Republic.

It would reach the central Bahamas by Sunday, headed toward the southeast Florida coast, forecasters said.

A tropical storm warning, telling residents to expect storm conditions within 24 hours, was issued for Antigua and Barbuda, St. Kitts and Nevis, Anguilla, Saba, St. Eustatius, St. Barthelemy and St. Martin.

Puerto Rico, a U.S. territory with about 4 million people, was under a storm watch, meaning possible storm conditions in 36 hours. The U.S. and British Virgin Islands were put under the same alert.

Forecasters said the storm had the potential to strengthen.

The second storm of the season, Tropical Storm Beryl, skirted the North Carolina coast and blew over Nantucket Island and other popular northeastern U.S. tourist playgrounds in July.

The first storm of what is expected to be a busy June to November hurricane season was Alberto, which moved ashore harmlessly in the Florida Panhandle in mid-June.

Forecasters have predicted up to 17 tropical storms and hurricanes this year. Last year saw a record 28, including Hurricane Katrina, the costliest natural disaster in U.S. history. It devastated New Orleans and other parts of the Gulf Coast and killed more than 1,300 people.