Homemade bombs and mines hidden in inner tubes or other harmless-looking flotsam may be bobbing in U.S. waters, a confidential FBI bulletin warned shortly before new anti-terror shipping laws take effect.

The FBI report, issued Wednesday and titled "Potential Indicators of Floating Improvised Explosive Devices or Terrorist Improvised Mines," said the agency knew of no specific planned attack, an FBI official said yesterday. But he said the bulletin sought to warn local, state and federal authorities that such attacks could occur.

"This is information that is disseminated on a weekly basis in order to provide law enforcement with current, relevant terrorism information," the official said. "This particular one dealt with the potential indicators of floating improvised explosive devices."

U.S. officials have repeatedly voiced concerns about possible seaborne attacks since the bombings of the USS Cole in 2000 and the French tanker Limburg in 2002, both off Yemen. Fears of militant activity in the United States have increased preceding elections in November.

The warning about possible homemade explosives in suspicious floating objects came about a week before a new United Nations security code designed to avert maritime terrorist attacks comes into force on Thursday.

The FBI official said he did not know if the bulletin was specifically timed to heighten awareness before the code takes effect.

The United States has vowed to enforce compliance with the strict new regulations. Its efforts are seen as a global litmus test of the new code's effectiveness.

The United Nations' new International Ship and Port Facility Security Code and the related U.S. Maritime Transportation Security Act, which ensures U.S. compliance with the U.N. regulations, are considered the most comprehensive security measures to be imposed on the maritime industry since World War II.

Beginning Thursday, all ships must obtain special security certificates that meet new International Maritime Organization standards.

The new code also stipulates that ports visited in the complex web of trade need to be security certified, with vessels required to keep a log of the last 10 ports they have visited.

Some industry experts have expressed concern that the U.S. pledge to force compliance could slow or harm world trade. The Coast Guard says it will board every ship on its first entry to the United States after Thursday to check compliance with the new rules.