Barack Obama rails against political 'strongmen'

The US president has frequently employed high-profile moments as opportunities to criticise Donald Trump

US President Barack Obama railed against political "strongmen", nationalism and isolation in a farewell address to the United Nations General Assembly, taking his campaign battle with Donald Trump to the podium of the world's foremost international institution.

Obama's speech in New York on Tuesday was a defense of globalism and an attack on authoritarians, tribalists and populists in which he never mentioned the Republican vying to succeed him in office, bloomberg.com reported.

But the subtext was impossible to miss, and could also be interpreted as a criticism of Britain's decision this year to exit from the European Union.

After extolling the expansion of democracy across the globe and a reduction in extreme poverty, Obama warned of "a crude populism, sometimes from the far left but more often from the far right, which seeks to establish what they think was a better, simpler age free from outside contamination."

"A country ringed by walls would only imprison itself," Obama said.

The remarks were the latest attack on Trump by an incumbent president who is extraordinarily invested in electing Democrat Hillary Clinton as his successor.

He has frequently employed high-profile moments in the final months of his administration as opportunities to criticise Trump and assist Clinton, his former secretary of state.

But Tuesday's speech also revealed a president still grappling with an argument for a more interconnected world and robust international institutions at a time when many voters on both sides of the Atlantic reject that philosophy.

"This is the paradox that defines the world today," Obama said, adding that the world could not afford to "dismiss these visions."

"They are powerful," he said. "They reflect dissatisfaction among too many of our citizens."