(Reuters) - Famed U.S. swimmer Michael Phelps, who recently returned to top-level competition after a two-year retirement, said on Sunday he would take a break from swimming and undergo treatment after his arrest last week for drunken driving.

Phelps, 29, who is the most decorated Olympian of all time, did not say how long a break he plans to take from the sport, though his agent said in a statement that he is entering a six-week, "comprehensive" inpatient program.

A winner of 22 Olympic medals, Phelps has appeared to be pursuing a spot on the U.S. team for the 2016 Olympics.

Phelps was arrested early on Tuesday after going 84 miles per hour (135 kph) in a 45-mph (72-kph) area, crossing the double-lane lines inside a Baltimore tunnel and subsequently failing a Breathalyzer test, according to police.

"The past few days have been extremely difficult," the 18-time Olympic gold medalist said on Twitter. "I'm going to take some time away to attend a program that will provide the help I need to better understand myself."

Phelps first tested the comeback waters in April at a low-key meet in Phoenix and his progress had been steady rather than his usual impressive.

He has had to deal with the unfamiliar sting of losing and re-calibrating his own sky-high standards.

But his results and times slowly improved through the U.S. nationals in August and his international return to competition at the Pan Pacific championships later that same month in Australia.

At the latter competition, he finished with gold medals in the 100 meters butterfly, the 4x200m and medley relays as well as silvers in the 200m individual medley and the 4x100m relays.

The 100m butterfly is one of two individual events that Phelps has won at three successive Olympics. If he qualifies for the event at the 2016 Rio Games, he will automatically make the U.S. men's medley relay team, which has never been beaten at the Olympics.

USA Swimming Executive Director Chuck Wielgus said the organization supports Phelps' decision to take a break from swimming in order to focus on his well-being.

"His self-recognition and commitment to get help exhibit how serious he is to learn from this experience," Wielgus said in a statement.

The drunken-driving arrest was the second for Phelps, who has spent most his life in the pool but admits he likes to have a good time when not practicing or competing.

Phelps was charged in Maryland in 2004 for drunken driving when he was 19. He pleaded guilty to a lesser charge of driving while impaired in exchange for 18 months' probation.

In 2009, British tabloids published a photo of Phelps smoking from a marijuana pipe while at a party at the University of South Carolina.

(Reporting by Jonathan Kaminsky in New Orleans and Mark Lamport-Stokes in Los Angeles; Additional reporting by Steve Ginsburg; Editing by Stephen Powell and Eric Walsh)