US President Barack Obama on Monday mourned the death of Margaret Thatcher, calling the first female British prime minister "a true friend" of the United States.

"With the passing of Baroness Margaret Thatcher, the world has lost one of the great champions of freedom and liberty, and America has lost a true friend," the president said in a statement.

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.

"As a grocer's daughter who rose to become Britain's first female prime minister, she stands as an example to our daughters that there is no glass ceiling that can't be shattered," said Obama, who has two teenage daughters.

"As an unapologetic supporter of our transatlantic alliance, she knew that with strength and resolve we could win the Cold War and extend freedom's promise," the president said.

"Here in America, many of us will never forget her standing shoulder to shoulder with President (Ronald) Reagan, reminding the world that we are not simply carried along by the currents of history -- we can shape them with moral conviction, unyielding courage and iron will," Obama added.