Updated 4.05pm

Robert Abela has joined world leaders in paying tribute to outgoing US President Joe Biden after he announced he was dropping out of the presidential race.

The prime minister said Biden had enjoyed a "remarkable career" and had impacted "numerous international issues amid a landscape of geopolitical challenges".

Earlier foreign minister Ian Borg said he was "grateful" to Biden for "his firm stance on increasing support to Ukraine as it fights for its freedom, as well as his continued efforts to bring an end to hostilities in the Middle East.”

Biden announced his decision to step down late on Sunday, a stunning move that upends the 2024 race for the White House. He endorsed Vice President Kamala Harris as the Democratic Party's new nominee.

Soon after the announcement, tributes poured from all over the world.

Ukrainian President Volodymyr Zelensky thanked Biden for taking "bold steps" in supporting his country, praising the US president's "tough but strong decision" to end his reelection bid.

"He supported our country during the most dramatic moment in history, assisted us in preventing (Russian President Vladimir) Putin from occupying our country, and has continued to support us throughout this terrible war," he said.

Polish Prime Minister Donald Tusk praised Biden for taking "many difficult decisions thanks to which Poland, America and the world are safer, and democracy stronger".

"I know you were driven by the same motivations when announcing your final decision. Probably the most difficult one in your life," added Tusk, who served as the European Council president between 2014 and 2019.

UK Prime Minister Keir Starmer said he respected Biden's decision."I know that, as he has done throughout his remarkable career, he will have made his decision based on what he believes is best for the American people," said Starmer.

German Chancellor Olaf Scholz also paid tribute to "my friend".

"Joe Biden has achieved a lot: for his country, for Europe, for the world," he said. "His decision not to run again deserves respect."

Israel's Defence Minister Yoav Gallant thanked Biden for his "unwavering support of Israel over the years".

He added: "Your steadfast backing, especially during the war, has been invaluable. We are grateful for your leadership and friendship."

Japan's premier said he recognised that Biden's announcement was one "he feels is the best political decision he can make".

"The Japan-US alliance is the cornerstone of our country's diplomacy and security, so we will monitor the situation closely," Prime Minister Fumio Kishida said.

Australian Prime Minister Anthony Albanese thanked Biden "for your leadership and ongoing service.

"The Australia-US alliance has never been stronger with our shared commitment to democratic values, international security, economic prosperity and climate action for this and future generations."

New Zealand's Prime Minister Christopher Luxon said Biden had "dedicated his life to public service, and that is something that deserves much respect".

Venezuela's President Nicolas Maduro told Biden "to be at ease with your conscience because you took the sensible and correct decision".

"He prioritised his family, his health and he realised at that age, with failing health, he couldn't take the reins of his country, much less a presidential candidacy," said Maduro, who is preparing for presidential polls this month.

Canadian Prime Minister Justin Trudeau also thanked Biden for his years of service."He's a great man, and everything he does is guided by his love for his country," he said.

"As president, he is a partner to Canadians -- and a true friend. To President Biden and the First Lady: thank you."

Former president Barack Obama, with whom Biden served two terms as vice president, praised his record in office.

"Internationally, he restored America's standing in the world, revitalised NATO, and mobilised the world to stand up against Russian aggression in Ukraine," he said.

While he had every right to run for re-election, Biden's decision to drop out of the race was a testament to his "love of country", Obama added.

Meanwhile the Kremlin said it was monitoring developments. 

"The election is still four months away. And it's a long time, during which a lot can change. We need to pay attention, follow what will happen and go about our business," Kremlin spokesman Dmitry Peskov told the Life.ru news outlet.

Chinese foreign ministry spokeswoman Mao Ning declined to comment, saying at a regular daily news conference: "The US presidential election is an internal affair of the US."

Even as world leaders hailed Biden's performance on the world stage, leading Republicans were insisting he was unfit to remain president.

"If Joe Biden is not fit to run for president, he is not fit to serve as president," said House Speaker Mike Johnson, the top Republican in Congress.

"He must resign the office immediately. November 5 cannot arrive soon enough," he added.

Former president Donald Trump, the Republican nominee, wrote on his Truth Social network: "Crooked Joe Biden was not fit to run for president, and is certainly not fit to serve."