(CNN) Two men have been charged this week with sexually abusing women seated near them on flights to Seattle.

The unrelated cases come at a time when the number of sexual assaults reported on planes has increased at "an alarming rate," according to the FBI.

The first incident was in January aboard a flight from London, and the second happened in March on a flight from Anchorage, according to federal prosecutors.

Babak Rezapour and Nicholas Matthew Stevens are accused of abusive sexual contact on an aircraft, which is punishable by up to two years in prison.

Rezapour appeared in US District Court in Los Angeles on Thursday. It's not clear whether a plea was entered. Stevens will appear in US District Court in Anchorage, Alaska, on Friday.

Court records did not indicate whether the men had attorneys. CNN was unable Thursday evening to reach either man.

What prosecutors say happened in January

On January 10, a woman on a Norwegian Air flight from London was moved by flight attendants to an aisle seat in the middle section of the plane after another passenger became ill, according to a complaint filed by federal prosecutors in Seattle.

Rezapour, 41, was in the other aisle seat, with an empty seat between them, the complaint says. It was not his assigned seat.

There was turbulence on the flight, so the woman took medication for nausea and anxiety and drank a glass of wine, according to federal prosecutors.

Later in the flight, Rezapour, of Van Nuys, California, whom the woman had never met, bought her another glass of wine.

"After drinking this second glass, VICTIM described feeling hazy and had difficulty remaining awake," the court filing states.

She kept falling asleep but awoke several times, startled, to find Rezapour sexually assaulting her, prosecutors said in a statement Thursday.

At one point he used his jacket to shield the activity, according to the complaint. An FBI agent said Rezapour groped the woman and put her hand on his genitals.

The woman fled to the back of the plane, where flight attendants found her on the floor in a fetal position, according to the complaint. She then told them what had happened.

Rezapour denied engaging in unwanted activity, the complaint says.

The complaint says that Rezapour's DNA was later found in the woman's underwear.

What prosecutors say happened in March

On March 8 on an Alaska Airlines flight from Anchorage, prosecutors say Stevens, 37, was sitting in an aisle seat in the same row as a 22-year-old woman, who was in the window seat. They had one seat between them.

Stevens began harassing the woman and she told him repeatedly to stop, the complaint filed by federal prosecutors said. He asked her to tell him some things she loved, according to the complaint, and she responded.

Then, the complaint says, he told her he loved "killing animals, killing more animals and going fishing so I can kill more animals." He then goes on to ask, "Who would I have to kill to get a girl like you?" and started joking around about killing people, the complaint says. The woman says she became scared of him.

At one point he moved closer to her and, while leaning his head on her shoulder, grabbed her thigh and breast several times, federal prosecutors said.

An airline employee was told about the incident and contacted police. The defendant told officers he and the woman had touched each other during the flight, according to the complaint.

FBI: What to do if it's happening to you

"We urge the flying public to report assaults to airline personnel and law enforcement and assist victims who need help," US Attorney Annette L. Hayes said in a statement. "The skies will be safer for all of us."

FBI investigations into midair sexual assaults increased by 66% from fiscal year 2014 to 2017. The bureau said it had opened 63 investigations into sexual assault on aircraft in 2017, compared with 57 in 2016, 40 in 2015 and 38 in 2014.

FBI Special Agent David Rodski told reporters in June the number of sexual assaults during flights is increasing "at an alarming rate," and added, "We're not sure why."

Last year, CNN reported it is difficult to determine just how frequently assaults happen on commercial flights because no federal regulatory agency tracks that data nationwide.

The FBI suggests passengers keep the armrest between them and a stranger down, and don't give them the benefit of the doubt.

"Offenders will often test their victims, sometimes pretending to brush against them to see how they react or if they wake up," the FBI warns.

If such behavior occurs, trust your gut, reprimand the person immediately and consider asking to be moved to another seat. And if an incident happens, report it to the flight crew immediately, and ask that they record the attacker's identity and report it.

"It doesn't matter when you report an in-flight sexual assault -- we take it seriously, and we will pursue it," says FBI Special Agent David Gates. "After the fact, these cases are much more difficult to prove."