The seven decades of Queen Elizabeth II's history-making reign bore witness to an age of unprecedented human advancement and the forward march of human dignity, United States President Joe Biden and First Lady Jill Biden said on Thursday, observing that the late monarch "defined an era".

Queen Elizabeth II, the United Kingdom's longest-serving monarch, died on Thursday at Balmoral Castle in Scotland after reigning for 70 years. She was 96.

Her death brings to an end the longest reign in the history of the United Kingdom, and one of the longest reigns by any head of state.

"In a world of constant change, 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. An enduring admiration for Queen Elizabeth II united people across the Commonwealth," Biden and the First Lady said in a statement.

The President said that the seven decades of her history-making reign bore witness to an age of unprecedented human advancement and the forward march of human dignity.

She was the first British monarch to whom people all around the world could feel a personal and immediate connection-whether they heard her on the radio as a young princess speaking to the children of the United Kingdom, or gathered around their televisions for her coronation, or watched her final Christmas speech or her Platinum Jubilee on their phones.

"And she, in turn, dedicated her whole life to their service," said the first US couple.

Supported by her beloved Prince Philip for 73 years, Queen Elizabeth II led always with grace, an unwavering commitment to duty, and the incomparable power of her example. She endured the dangers and deprivations of a world war alongside the British people and rallied them during the devastation of a global pandemic to look to better days ahead, the presidential statement said.

"Through her dedication to her patronages and charities, she supported causes that uplifted people and expanded opportunity. By showing friendship and respect to newly independent nations around the world, she elevated the cause of liberty and fostered enduring bonds that helped strengthen the Commonwealth, which she loved so deeply, into a community to promote peace and shared values, it said.

Biden and the First Lady said that the queen was a stateswoman of unmatched dignity and constancy who deepened the bedrock Alliance between the United Kingdom and the United States. She helped make the US-Britain relationship special.

"She stood in solidarity with the United States during our darkest days after 9/11, when she poignantly reminded us that Grief is the price we pay for love," the statement said.

"In the years ahead, we look forward to continuing a close friendship with The King and The Queen Consort. Today, 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. We send our deepest condolences to the Royal Family, who are not only mourning their Queen, but their dear mother, grandmother, and great-grandmother. Her legacy will loom large in the pages of British history, and in the story of our world, said Biden and the First Lady.

The White House, mourning the death of Queen Elizabeth II, said that its "hearts and thoughts go to the family members of the Queen, go to the people of the United Kingdom.

White House Press Secretary Karine Jean-Pierre told reporters: Our relationship with the people of the United Kingdom, and this is something that the President has said himself, has grown stronger and stronger. The United Kingdom is one of our closest allies. And again, our hearts go to the people of the United Kingdom, to the Queen and to her family."

Former president Barack Obama said mourned the loss of the departed leader, observing that her reign was defined by grace, elegance, and a tireless work ethic, defying the odds and expectations placed on women of her generation.

"Her Majesty was just 25 years old when she took on the enormous task of helming one of the world's great democracies. In the decades that followed, she would go on to make the role of Queen her own -- with a reign defined by grace, elegance, and a tireless work ethic, defying the odds and expectations placed on women of her generation," Obama said in a statement.

From the day of her coronation 70 years ago - the first one ever televised - to this very moment, as countless tributes are being posted online in her honor, Queen Elizabeth II has captivated the world, he said. "Today, Michelle and I join so many others who are celebrating her life and mourning her passing, he said.

During World War II, she became the first-ever female royal to serve on active military duty. And through periods of prosperity and stagnation -- from the moon landing, to the fall of the Berlin Wall, to the dawn of the digital age -- she served as a beacon of hope and stability for the people of the United Kingdom and the world, said the former US president.

Obama said the queen worked with 15 Prime Ministers and countless foreign heads of state. She listened deeply, thought strategically, and was responsible for considerable diplomatic achievements. "And yet, she wore her lofty titles with a light touch -- as willing to act in a comic sketch for the London Olympics as she was to record steadying messages for the people of the UK during the COVID-19 lockdowns," he said.  

"Michelle and I were lucky enough to come to know Her Majesty, and she meant a great deal to us. Back when we were just beginning to navigate life as President and First Lady, she welcomed us to the world stage with open arms and extraordinary generosity. Time and again, we were struck by her warmth, the way she put people at ease, and how she brought her considerable humor and charm to moments of great pomp and circumstance," he said.

"Like so many, Michelle and I are grateful to have witnessed Her Majesty's dedicated leadership, and we are awed by her legacy of tireless, dignified public service. Our thoughts are with her family and the people of the United Kingdom at this difficult time," he said.