NFL: Super Bowl XLIX-New England Patriots vs Seattle Seahawks Thomson Reuters Super Bowl 50 will be one of the most highly guarded sporting events in U.S. history as security services pile resources into preventing any repeat of the deadly attacks in Paris and San Bernardino, although officials have said there is no specific or credible threat to Sunday's game in the San Francisco Bay Area.

November's attacks in the French capital by gunmen linked to the Islamic State militant group included suicide bombings near entrances to the national stadium as a soccer game was underway, provoking fears of attacks targeting crowds attending big sporting events elsewhere.

Federal U.S. security officials are tight-lipped as to how their plans differ for this year's National Football League (NFL) championship game from previous Super Bowls, where security is always tight.

But hundreds of U.S. law enforcement agents are moving into the Bay Area in the week leading up to the game, to be held at Levi's Stadium in Santa Clara, some 50 miles (80 km) southeast of San Francisco.

The U.S. Federal Bureau of Investigation (FBI) will be deploying commando-style SWAT (special weapons and tactics) teams, officials said, as well as bomb experts and evidence collection technicians so that specialists are in place should the worst happen.

"This is a high-profile target," said David Johnson, special agent in charge of the FBI's San Francisco field office. "A terrorist group would receive a great deal of publicity (if they attacked it), which is what they are looking for."

The game is expected to attract more than 100 million viewers in the United States and millions more around the world.

The FBI will have helicopters and planes in the air over the stadium, Johnson said, and security will be tight at game-related venues such as an NFL exhibit in downtown San Francisco and practice fields used by the teams in the week leading up to the game.

Levi's Stadium in San Francisco, where Super Bowl 50 will be played February 7. Getty Images/Thearon W. Henderson

Officials with the Department of Homeland Security will set up a giant X-ray machine to screen all cargo going into the stadium, among other tasks, spokeswoman Marsha Catron said.

The Transportation Security Administration is sending "VIPR teams" (visible intermodal prevention and response) that include so-called behavior detection officers to watch over crowds, she added, especially at busy locations such as train stations.

The Super Bowl is considered the second-highest threat environment under U.S. federal guidelines, a notch below what is called a National Security Special Event, which includes summits of world leaders and presidential inaugurations.

Federal and local law enforcement agencies issued a bulletin this month outlining theoretical scenarios, including "lone wolf" attackers like December's shooting rampage in San Bernardino, California.

The most vulnerable "targets of opportunity," it concluded, are outside, not inside, the stadium.

Two federal officials said a no-fly zone will effectively be imposed over the stadium and its environs as a precaution, with traffic through it largely, if not exclusively, limited to official aircraft.

Because the game venue sits by a creek linked to the Bay, U.S. Coast Guard boats will create a maritime exclusion zone around it, too, the officials said.

Police are also bracing for possible street protests mentioned on social media by civil rights activists.

"There is a delicate balance and we don't want to infringe upon anyone's constitutional rights," Santa Clara Police Lieutenant Kurt Clarke told Reuters. "We want to provide a safe Super Bowl and we want folks to be lawful."

Additional reporting by Curtis Skinner in San Francisco. Writing by Daniel Wallis. Editing by Bill Rigby.