More than 100 U.S. Navy instructor pilots are refusing to fly in protest of what they say is the refusal of top brass to adequately address an urgent problem with training jets' oxygen system, multiple instructor pilots tell Fox News.

The boycott started late last week and has effectively grounded hundreds of training flights.

"The pilots don't feel safe flying this aircraft," one instructor pilot told Fox News.

Among the hundreds of student pilots affected is Marine 1st Lt. Michael Pence, son of Vice President Pence -- a factor that could put added pressure on the Pentagon to resolve the dispute.

Vice Adm. Mike Shoemaker, head of naval aviation, told Fox News in an exclusive interview that the training jet issue is the "number one safety priority" across naval aviation right now.

"Right now we don't have the smoking gun," he cautioned.

In the last five years, physiological episodes, caused in part by problems with the oxygen system, have nearly quadrupled on the T-45 training jet, according to Capitol Hill testimony last week by senior naval aviators.

"There is no question that there are problems that are being covered up," House Armed Services Committee Chairman Mac Thornberry, R-Texas, said. "I am very concerned about the issue. It's been getting worse over time and if you look at the statistics, the older airplanes are having bigger problems than newer airplanes."

Sen. John McCain, R-Ariz., said in a statement that the Navy "must address these safety concerns swiftly and decisively."

Fox News interviewed multiple Navy flight instructors who say incidents of oxygen poisoning in the 30-year old T-45 Goshawk have "skyrocketed."

"Histotoxic hypoxia" is the medical term associated with the disorientating disorder which can put pilots' lives at risk, as well as those of civilians on the ground below. Two instructor pilots say the training jets are now averaging three incidents a week, as the Navy struggles to get to the bottom of the contamination.

"It can happen without warning," one pilot said. "The system doesn't detect contaminants."

A number of instructors cited recent episodes as reasons for the abrupt work stoppage.

Last week, a student from training squadron VT-86 in Pensacola, Fla., had to be "dragged out" of his jet because he became "incapacitated" from the faulty oxygen system, according to two flight instructors.

In March, a British exchange instructor pilot with thousands of hours in the cockpit had to conduct an emergency landing during a training flight near Meridian, Miss., after both he and his student experienced hypoxic symptoms.

In August, a flight instructor and his student were forced to eject near Kingsville, Texas, when they felt symptoms of hypoxia, crashing the multi-million dollar jet. Both pilots ejected safely and were not seriously injured.

Last month, there were 10 episodes in T-45s, according to Shoemaker.

Anticipating the pilot protest, the Navy sent a team of engineers and other specialists this week to its T-45 training bases in Kingsville, Meridian and Pensacola for talks with the pilots.

A meeting Tuesday in Meridian "got heated," Fox News is told. The pilots told the civilians from Navy Air Systems Command their complaints about the oxygen system were being ignored. When a senior Navy pilot showed photos of a faulty oxygen system he claimed had been sent up to NAVAIR's headquarters in Maryland, the engineers said they never received the photos.

Asked about the protest, Navy spokeswoman Cmdr. Jeannie Groeneveld acknowledged that 40 percent of instructor pilots refused to fly their training flights Friday. A flight instructor said the number was closer to 75 percent, because the Navy reduced the flight schedule knowing more than half the pilots would refuse to fly.

Shoemaker said following a meeting in Kingsville, flights there had resumed.

Just last week, the vice president's wife Karen Pence visited her son at Naval Air Station Meridian, where her son is in flight training. A source told Fox News she told the commodore in charge of flight training, "Take care of my son" -- and a day later the senior Navy officer told a group of instructor pilots he feared he was unable to do that because of the danger posed by the faulty oxygen system.

Across the Navy, training squadron skippers and commodores are telling their instructors to follow their instincts -- and not forcing them to fly -- much to the appreciation of those boycotting until they feel the issue is resolved, Fox News is told.

"There is not a, you know, 'Fly or else,'" Shoemaker said.

Some instructor pilots point to Rear Adm. Dell D. Bull, chief of naval air training, as the culprit in ignoring the unsafe conditions.

"He is telling us to just, 'shut up and color,'" one pilot said.

Some instructor pilots have refused to go along with the boycott and continue flying, but remove the oxygen masks as soon as they take off.

"Most pilots think [Bull] is just dragging his feet and doesn't want to look bad," said one pilot, who like the others declined to be named because they were not authorized to speak to the press.

"It's the admirals … the people that have the power to fix it that aren't doing a damn thing," one frustrated instructor pilot said.

Bull is planning to speak to the instructor pilots in Meridian on Wednesday. Shoemaker is preparing a video message to present to the pilots who still refuse to fly.

Two senior Navy officials interviewed by Fox News say the Navy is doing its best to tackle the problem.

"We have been working this for five, six years now to try to get to the bottom of this," one official said.

Both officials acknowledged "communication problems" between the upper echelon of the Navy and the instructors.

Six months ago, the Navy sent the T-45 and other jet squadrons Sorbent tubes to measure the air the pilots were breathing. After each flight, the tubes were sent to a lab in Maryland for analysis. After 1,500 flights worth of air samples, the results remain inconclusive.

"We haven't come up with anything conclusive … showing a contaminant or something like that," a senior official said.

The instructor pilots see it differently.

"They sent our squadron six tubes," one pilot said. "That's part of the frustration. They are doing the absolute minimum."

The senior Navy officials say they understand the pilots' frustration and will soon issue more advanced hydrocarbon sensors which can by analyzed on-site, speeding up the investigation.

The dangers with the oxygen system are not limited to the T-45 training jets either. U.S. Navy F/A-18 Hornets have been known to suffer similar problems.

Just weeks after Fox News visited the USS George H.W. Bush off the coast of North Carolina before it deployed to the Middle East, three F-18 pilots had to be flown out after suffering from hypoxia, forcing them into a decompression chamber. Today in the Persian Gulf while striking ISIS, the aircraft carrier has a special decompression machine on board to help in the event of future emergencies.

Lucas Tomlinson is the Pentagon and State Department producer for Fox News Channel. You can follow him on Twitter: @LucasFoxNews