At least 35 Greek ships are monitoring the Ionian Sea along Greece's west coast, the Aegean Sea and areas off the coasts of Crete and the resort Cyclades islands.
The ships are checking "any ships with suspicious behavior," a Navy source told The Associated Press on condition of anonymity.
About 70 vessels have been inspected by Greek forces in recent weeks, but there have been no indications of cargo or personnel considered as possible threats to the Aug. 13-29 Games, the source said.
The inspections will increase as the games approach, the source added. About 10 Navy helicopters and two military patrol aircraft are assisting the ships.
NATO plans to dedicate its entire Mediterranean fleet of about 15 vessels to Olympic patrols in international waters. The NATO force has been on anti-terrorism duty since shortly after the Sept. 11, 2001, attacks on New York and Washington.
Maritime protection has taken a prominent role in Greece's $1.5 billion security network. Greece — with a coastline of 9,320 miles and some 6,000 islands and islets — is a major destination along illegal smuggling routes for immigrants and goods.
Some global security analysts warn that al Qaeda or other terrorist groups could be studying possible attacks on shipping lanes and other sea targets.
More than 200 naval commandos are stationed at seaside Olympic venues. Off the port of Piraeus, about six miles south of central Athens, a frigate with about 200 sailors will watch over at least eight cruise ships that will serve as floating hotels for heads of state and other dignitaries. The first of the cruise ships is expected next week.
By Lisa Orkin