Jimmy Carter "lived a life measured not by words, but by his deeds," President Joe Biden said on Sunday in televised remarks addressing the death of the former US leader.

Biden's tribute, like many that came in from US and global leaders, focused on Carter's character, with the outgoing president describing his predecessor as seeming to be from a "bygone era."

"Just look at his life, his life's work," Biden said, speaking from the US Virgin Islands, where he was on vacation.

"He worked to eradicate disease, not just at home, but around the world. He forged peace, advanced civil rights, human rights, promoted free and fair elections around the world. He built housing for the homeless with his own hands."

Biden also declared January 9 a national day of mourning for Carter, calling on Americans to visit their places of worship to "pay homage" to the late US leader.

"I call on the American people to assemble on that day in their respective places of worship, there to pay homage to the memory of President James Earl Carter, Jr," Biden said in a White House proclamation.

"I invite the people of the world who share our grief to join us in this solemn observance."

Among the former US presidents, Bill Clinton said his predecessor had "worked tirelessly for a better, fairer world."

George W. Bush said Carter "dignified the office. And his efforts to leave behind a better world didn't end with the presidency."

Barack Obama hailed Carter for teaching "all of us what it means to live a life of grace, dignity, justice, and service."

In Europe, British Prime Minister Keir Starmer paid tribute to Jimmy Carter for his "lifelong dedication to peace" and said the former US president had "redefined the post-presidency with a remarkable commitment to social justice and human rights".

Ukrainian President Volodymyr Zelensky said that Jimmy Carter served as US president when Ukraine was not yet independent but "his heart stood firmly with us in our ongoing fight for freedom".

"We deeply appreciate his steadfast commitment to Christian faith and democratic values, as well as his unwavering support for Ukraine in the face of Russia's unprovoked aggression."

In Cuba, the island's leader Miguel Diaz-Canel said on Sunday his nation would remember Carter "with gratitude".

During his presidency, Carter made moves to normalise diplomatic ties and relax US sanctions on the country, and "our people will remember with gratitude his efforts to improve relations," Diaz-Canel said in a post on social media.

Elsewhere, Panama's President Jose Mulino said negotiations with Carter "achieved the transfer of the Canal to Panamanian hands and the full sovereignty of our country."

US President-elect Donald Trump, who will return to the White House in January, has threatened to demand the key waterway be returned to the United States, which originally completed and operated it. (AFP)