President Joe Biden said in a statement on Queen Elizabeth II's death that "the thoughts and prayers of people all across the United States are with the people of the United Kingdom and the Commonwealth in their grief."
"Her Majesty Queen Elizabeth II was more than a monarch," Biden and first lady Jill Biden said Thursday in a written statement, shortly after the queen's death was announced. "She defined an era."
"In a world of constant change," they continued, "she was a steadying presence and a source of comfort and pride for generations of Britons, including many who have never known their country without her."
The Bidens said the late queen helped make the U.S.-U.K. relationship "special."
"Queen Elizabeth II was a stateswoman of unmatched dignity and constancy who deepened the bedrock alliance between the United Kingdom and the United States," they wrote.
Senior advisers informed Biden of the queen's death during a meeting in the Oval Office, according to a White House official.
He ordered flags flown at half-staff at the White House and on all public U.S. buildings around the world until sundown on the day her body is laid to rest. After her funeral, her body is expected to be buried at Windsor Castle outside London.
The Bidens said they looked forward "to continuing a close friendship" with her son, now King Charles III, and his wife, Camilla, the queen consort, "in the years ahead."
"We send our deepest condolences to the Royal Family, who are not only mourning their Queen, but their dear mother, grandmother, and great-grandmother," they said. "Her legacy will loom large in the pages of British history, and in the story of our world."
The Bidens subsequently stopped at the British Embassy in Washington, D.C., to offer their condolences to British Ambassador Karen Pierce and the staff there, signing a book and making some brief comments.
"I mourn for all of you," President Biden said as he stood up from signing the condolence book. "She was a great lady. We're so delighted we got meet her."
At one point, the president noted the queen was very interested in American domestic politics and said, "She was quite a lady."
"We admired her so much," Jill Biden said.
"It's amazing the impact she had not just to the Commonwealth, but around the world. Around the world," the president added.
When the first couple entered embassy late Thursday afternoon, the first lady was clutching a bouquet. Ambassador Pierce greeted them near the door.
Pierce invited President Biden to sign the condolence book and he replied, "Am I able to write a message?" He proceeded to write at length in the book including that the queen "led with enduring strength and dignity, devoting her whole life to serving her people. Jill and I will never forget meeting her." The first lady added her own brief message in the book as well.
The news of the death of the queen, who was 96, broke earlier Thursday as White House press secretary Karine Jean-Pierre was holding a regular news briefing. Jean-Pierre learned of Buckingham Palace's announcement from reporters.
"Our hearts go to the people of the United Kingdom, to the queen and to her family," Jean-Pierre said.
At the Capitol, House Speaker Nancy Pelosi also ordered flags lowered to half-staff.
"Today, Americans join the people of the United Kingdom in mourning the sad passing of Queen Elizabeth II," Pelosi tweeted. "Over her seven decades on the throne, Her Majesty was a pillar of leadership in the global arena and a devoted friend of freedom."
Earlier Thursday, Biden spoke with British Prime Minister Liz Truss about the queen and told Truss he was "thinking very much" about the ailing monarch, the White House said.
On a pre-scheduled call with European leaders about Ukraine, which White House spokesman John Kirby told reporters was ongoing early Thursday afternoon, the president said he and first lady Jill Biden were "thinking" of the British royal family.
"He did convey to Prime Minister Truss, who is on the video teleconference, that he and Dr. Biden are thinking very much of the queen and the family and the people of the United Kingdom," Kirby said.
Biden has been briefed on developments and "will be updated throughout the day concerning news out of the United Kingdom," Kirby said.
"His and the first lady's thoughts are solidly and squarely with the queen today, and her family," Kirby said.
Buckingham Palace has said earlier Thursday that Queen Elizabeth's doctors were "concerned for" her health, and had "recommended she remain under medical supervision."
Queen Elizabeth met 13 sitting U.S. presidents in her lifetime, and met every U.S. president since World War II, with the exception of Lyndon B. Johnson.
Biden met with the queen last year during a trip to the United Kingdom -- his first travel abroad as president.
He had previously met the queen in 1982 as a U.S. senator, during another trip to the U.K.
ABC News' Sarah Kolinovsky and Allison Pecorin contributed to this report.