US Secretary of State John Kerry on Monday mourned the death of former British Prime Minister Margaret Thatcher, hailing her as "a dear friend" of the United States and a "transformative leader" in the world.

"I was saddened to learn of Baroness Thatcher's passing, and Teresa and I join millions across the United Kingdom and around the world in mourning her loss and celebrating her life," Kerry said in a statement issued by the State Department.

Thatcher died of a stroke on Monday at the age of 87. She held the premiership from 1979 to 1990 and was renowned as "Iron Lady" worldwide.

"The United States has lost a dear friend, and the world has lost a transformative leader who broke the glass ceiling in global politics," said the US chief diplomat. "With her characteristic determination, she defined grit on the world stage and grace in the face of advancing age and illness."

Kerry also paid tribute to Thatcher's contribution in strengthening the "special relationship" between the United States and the United Kingdom.