The White House dog Major has bitten again, just days after returning from training in Delaware following a similar incident earlier this month.
A spokesman for First Lady Jill Biden said that "out of an abundance of caution" an individual was seen by medical staff before returning to work.
Major is the younger of the Bidens' two German Shepherds and the White House's first ever rescue dog.
"He's a sweet dog," President Joe Biden has said.
Michael LaRosa, Mrs Biden's spokesman, said of Monday's incident: "Major is still adjusting to his new surroundings and he nipped someone while on a walk."
CNN reported that a National Park Service employee was bitten on the South Lawn and had to stop work to receive treatment. the National Park Service has not responded to CNN's request for comment.
Both Major and Champ, the Biden's 13-year-old dog, were moved to the Biden family home in Wilmington, Delaware, after the first biting incident on 8 March.
Anonymous sources told CNN at the time that Major had been jumping, barking and charging at White House staff and security.
President Biden said Major was acclimatising to so many new people around him. "You turn the corner and there's two people you don't know at all and they move, and he moves to protect," he told ABC's Good Morning America.
He went on to say: "Eighty-five per cent of the people [at the White House] love him. All he does is lick them and wag his tail. But... I realise some people, understandably, are afraid of dogs to begin with."
The Bidens adopted Major, who is now three years old, as a puppy from Delaware Humane Association in 2018. Champ had already spent some time in the White House as a younger dog, when Mr Biden was vice-president, before moving in permanently in January this year.
Joe and Jill Biden's German shepherds have been removed from the White House after the younger dog, Major, reportedly bit a security agent.
According to US media, the pets have been sent back to the Biden family home in Wilmington, Delaware.
It follows Major's aggressive behaviour towards White House staff.
The Bidens adopted three-year-old Major in 2018. He became the first dog from an animal shelter to live in the White House.
Their other dog, Champ, is 13.
Anonymous sources told CNN that Major had been jumping, barking and charging at White House staff and security.
His wife, Jill, told The Kelly Clarkson Show last month she was focused on getting the dogs settled.
"They have to take the elevator, they're not used to that, and they have to go out on the South Lawn with lots of people watching them. So that's what I've been obsessed with, getting everybody settled and calm," she said.
Sharing the limelight with their owners, the dogs appeared alongside the first lady in a public service announcement encouraging Americans to wear face coverings, and also gave their own Christmas message in December.
The White House has a long history with pets, although Donald Trump was the first president in more than 100 years not to have a dog. At a rally in Texas in 2019, he said he would "feel a little phony" about walking one on the White House lawn, plus he didn't have the time.
Major is the first dog from a shelter to live in the White House, however President Lyndon B Johnson also had a rescue dog - a terrier mix named Yuki.
Yuki was found at a Texas petrol station by the president's daughter, Luci, on Thanksgiving Day in 1966, according to the Presidential Pet Museum website, which compiles information on the White House pets.
President Joe Biden's dogs have arrived at the White House, restarting a long-held tradition of having pets there.
Donald Trump was the first US president in more than 100 years not to have a pet.
Mr Biden's two German Shepherds - Champ and Major - joined the president and first lady over the weekend and seemed at home on the White House lawn.
The dogs have a big fan base on social media. A Twitter page in their name has tens of thousands of followers.
The pair were pictured at the White House on Sunday, a few days after their owners moved in.
"Champ is enjoying his new dog bed by the fireplace and Major loved running around on the South Lawn," a statement from the White House said.
Here's a look at what we know about the new "First Dogs", and their predecessors.
Champ and Major
Mr Biden got Champ as a puppy after being elected vice-president in 2008.
His wife, Jill, promised him the post-election pet, and posted pictures of different dogs on the back of the seat in front of him on his campaign plane, according to Politico.
The name, chosen by Mr Biden's grandchildren, had sentimental significance.
In campaign speeches for the 2008 election, he recalled how his father would tell him: "Any time you get knocked down, champ, get up!"
Major was fostered and then adopted in 2018 from the Delaware Humane Association.
Bo and Sunny
Portuguese Water Dogs Bo and Sunny were residents of the White House during Barack Obama's presidency.
When he declared victory, Mr Obama told his daughters: "You have earned the new puppy that's coming with us to the White House."
Bo was given as a gift to the Obama children from Senator Ted Kennedy in 2009, while Sunny came along in August 2013.
Bo, who has a white chest and front paws, and the all-black Sunny proved popular, even helping with official duties.
"Everybody wants to see them and take pictures," Michelle Obama said during her husband's presidency.
"I get a memo at the beginning of the month with a request for their schedules, and I have to approve their appearances."
Buddy and Socks
Bill Clinton had a chocolate Labrador called Buddy during his time as president, as well as a cat named Socks.
The pair often fought and the New York Times jokingly called them nemeses.
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