Mass shootings from Newtown to San Bernardino are weighing ever more heavily on Americans, with signs of post-traumatic stress spreading far beyond the circle of survivors and loved ones, experts say.

Gun violence kills about 30,000 Americans every year and mass shootings -- extremely rare in most countries -- have been on the rise in the United States.

According to tracking website gunviolencearchive.org, there were 330 mass shootings in the United States last year, up from 281 in 2014. They affected nearly every part of the country, reaching into both big cities and small towns.

When you add up the dead, the wounded, emergency personnel, relatives and other loved ones close and far, these massacres have "an impact on all of us," said Merritt Schreiber, a psychology professor at the University of California, Irvine.

The searing images of the young and innocent dying before their time have become harder to escape, especially as offices, hospitals and even elementary schools have begun holding regular "active shooter" drills.

That, experts say, has led to a rise in anxiety, depression and exhaustion, all the symptoms of Post-Traumatic Stress Disorder.

"Has America become a PTSD nation?" George Washington University psychology professor Jean Kim asks on the AlterNet website.

President Barack Obama, in announcing limited gun control measures Tuesday, wiped away tears as he remembered 20 elementary school children -- some as young as five or six -- shot dead three years ago in Newtown, Connecticut.

"We do not have to accept this carnage as the price of freedom," he said.

Obama formally unveiled a handful of executive measures intended to make it harder to buy and sell weapons.

But many Republicans immediately expressed opposition to the measures -- though Donald Trump conceded that he thought Obama's tears were sincere -- and even the president admitted that the new steps would not stop the scourge of mass shootings.

Indeed, on the very first day of this year, one such event in Texas wounded four people.

- Climate of fear -

With every passing incident, images of the dead and wounded carried out on stretchers, loved ones breaking down, pictures of the killers and the dark faces of authorities permeate television screens and 24-hour news channels.

"There is an emerging scientific evidence that suggests that spending a lot of time watching this sort of incident entails anxiety for adults and children," Schreiber said.

This, specialists say, translates into hyper-vigilance, a type of permanent state of alert close to paranoia, like the fear of sitting in a restaurant with your back to the door.

The repeated cycles of violence "perpetuate fearfulness and can create these divisions between us and the others," explained Eric Bergemann, a Los Angeles psychologist.

"When those things are repeated by the media, we get more and more scared because we are continuously reminded about those things that are out of our control."

Bergemann said that the traumatized then sometimes turn their focus on perceived enemies, in order to feel like they are doing something to avert danger.

Anti-Muslim attacks have been on the rise since a Muslim couple shot 14 people to death and injured 22 others in the California city of San Bernardino last month.

The climate of fear is powerful.

Bergemann's clients tell him, "I'd like to do this event, but I'm concerned about being in a public place" because an attack could take place there, he said.

- 'Kill drill' -

In many offices, employees are taught procedures to follow in case of an active shooter situation.

In one Los Angeles business building, notices repeat the sheriff department's message that you have three options during an active shooter situation: Run, hide or fight.

Many schools also hold "kill drills," where children learn how to hide in case a shooter comes onto school grounds.

"To some degree to kids who grow up like that, it's gonna be like earthquake drills, like it's normal, and that's kind of sad," said Catherine Mogil of the UCLA Semel Institute for Neuroscience and Human Behavior.

"This generation may end up having a higher level of anxiety or hyper-vigilance like our military kids," added Mogil, a licensed clinical psychologist.

In Roseburg, Oregon, "the whole community is on edge... kids are affected as well," said deputy fire chief Robert Bullock.

Three months ago, a mentally unstable student killed an assistant professor and eight fellow students with an assault rifle in that usually peaceful community.

Firemen, nurses and police officers are still under "a lot of post-traumatic shock," Bullock told AFP.

He pointed to "sleeping problems" and other difficulties.

"There's people, their patience is very short, some people are on an emotional roller coaster. One minute they're fine, the other, very emotional," Bullock added.

"Certain words make their hair stand up because they think they heard, 'shooting...' I don't think any of them will completely heal."