By Joseph Ax

NEW YORK (Reuters) - Millions of U.S. travelers flying during the busy Fourth of July holiday weekend will face heightened security and increased delays due to the deadly attacks at Istanbul's main airport, officials and air security experts said on Wednesday.

Airport officials were hesitant to reveal specific safety measures taken following Tuesday's attacks by suspected Islamic State militants, which killed 41 people and wounded 239 at Europe's third-busiest airport, but increased vigilance appeared to have resulted in at least one airport disruption.

A terminal at John F. Kennedy International Airport in New York was briefly evacuated on Wednesday morning while police investigated a report of a suspicious package.

The implementation of stricter security measures will likely increase travel time this weekend, air security experts said, even as the Transportation Security Administration continues to struggle amid personnel shortages.

"If you are in a 'marquee' airport, you should absolutely allow significantly more time, on the order of 30 to 45 minutes," said Bruce McIndoe, the chief executive officer of travel risk advisory company iJet International.

Authorities can "dial up" various security elements, from increasing the frequency of "random" passengers selected for extra screening to turning up the sensitivity of magnetometer devices, according to McIndoe.

Following the Istanbul attacks, which took place outside security checkpoints, U.S. airports are likely to focus on surveillance and armed personnel in similar public spaces not subject to screening, McIndoe said.

The Port Authority of New York and New Jersey, which oversees operations at the three major airports in the New York City area, said in a statement that police had added "high visibility patrols with tactical weapons and equipment."

The agency said it had already put in place counterterrorism patrols at various transportation hubs following the mass shooting in Orlando earlier this month.

Agencies in charge of other major airports, including Reagan and Dulles in the Washington, D.C. area, Logan in Boston, O'Hare in Chicago, Hartsfield-Jackson in Atlanta and Dallas/Fort Worth, declined to offer operational details but emphasized that security remains their top priority.

"Logan maintains an enhanced security posture," said an spokeswoman for the Massachusetts Port Authority. "There are many elements that are seen and unseen."

The security measures are not limited to airports. New York City Police Commissioner William Bratton told reporters on Wednesday that there will be more officers, including a counterterrorism unit, present at the city's July 4th celebrations.

Meanwhile, Amtrak said it had "robust security measures" in place and was working with other agencies to gather intelligence following the Istanbul attacks.

A record number of Americans, 43 million, are expected to travel between June 30 and July 4, according to AAA.

The vast majority will go by car, AAA said, but 3.3 million are expected to fly. That is more than 25 percent higher that the 2.6 million AAA projected to fly during Memorial Day weekend in May, after months of widespread complaints about long security lines.

The attacks in Istanbul, as well as bombings at Brussels' airport that also struck outside checkpoints, have reignited debate over whether airport screening should extend into public spaces, despite the increased inconvenience and questions about the effectiveness of such a move.

But McIndoe said those proposals lead to an "infinite loop" that has no solution; checking vehicles before they enter the airport, for instance, simply forces cars to queue up, creating a new target.

Despite the spectacular massacres, he added, the chance of dying in an attack while traveling by plane is infinitesimal, given the more than 3 billion passengers that fly each year.

"You're tens of thousands of times more likely to die in an automobile accident," he said.

(Additional reporting by Gina Cherelus and Laila Kearney in New York, Ian Simpson in Washington, D.C., and Lisa Baertlein in Los Angeles; Editing by Tom Brown)