(Adds Cayo Arcas reopening, updates Gustav strength)

MEXICO CITY, Sept 1 (Reuters) - The Mexican port of Cayo Arcas on the Gulf of Mexico coast, one of the country's three main oil exporting ports, reopened on Monday after being closed the previous day due to rough weather.

All other Mexican ports including Dos Bocas and Coatzacoalcos, the other two key crude ports in the Mexican Gulf, where Hurricane Gustav blew strong winds and rain into Louisiana, were also open, the communications and transport ministry said on its website.

Together the three Gulf oil ports ship about 80 percent of Mexico's crude exports, most of which goes to U.S. refineries.

Gustav weakened to a Category 1 storm on Monday after roaring ashore in Louisiana near a key logistical port that supports many U.S. drilling operations in the Gulf of Mexico.

Mexico is the world's No. 10 exporter of crude oil, according to the U.S. Energy Information Administration, and a major U.S. supplier.

State oil monopoly Pemex says its export volumes are rarely hurt by temporary port closures, as it reschedules delayed shipments once the weather clears. (Reporting by Catherine Bremer and Noel Randewich, editing by Matthew Lewis)

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