WASHINGTON -- Despite miles of new concrete barriers, towering walls, bomb-residue tests, metal detectors, gas masks and tight restrictions on diplomats' travel, a recent tunnel found near the U.S. Embassy in Rome makes clear that America's overseas missions remain strikingly vulnerable.

They also remain a target of choice.

Since Sept. 11, officials have uncovered plots to attack U.S. embassies or consulates in Paris and Sarajevo, Bosnia, in Turkey, Lebanon and Yemen, and last week in Rome.

"We're at a very high state of alert. We're taking all possible precautions," said State Department spokesman Richard Boucher.

A fresh sign of the potential risks came Saturday in Macedonia where police killed seven gunmen who were suspected of planning attacks on embassies in the capital of Skopje. A State Department official said the agency could not confirm that the U.S. Embassy was targeted.

After truck bombs simultaneously tore apart two U.S. embassies in Africa in 1998, the United States spent $4.3 billion to fortify its overseas outposts from attack.