The Chinese premier made no explicit reference to US President Donald Trump as he addressed the UN General Assembly. But he cast the Asian power as a defender of the global order, of which Washington until recently was the chief custodian.

"The world has entered a new period of turbulence and transformation," Li said.

"Unilateralism and (the) Cold War mentality are resurfacing. The international rules and order built over the past 80 years are under serious challenge and the once effective international system is constantly disrupted," he said, adding that humanity had "once again come to a crossroads."

Li criticized the imposition of tariffs, which Trump has seized on as a key tool to pressure countries, even though he has reached a truce with China.

"A major cause of the current global economic doldrums is the rise in unilateral and protectionist measures such as tariff hikes and the erection of walls and barriers," Li said.

"China has consistently opened its door wider to the world."

Li said that China "hopes to work with the rest of the world to uphold the ideals of the UN."

The United States and its Pacific allies have long demanded that China accept the status quo and allow freedom of navigation in the strategic South China Sea, where Beijing has myriad disputes.

The Trump administration, while still critical of China, has shifted away from defending international conventions and instead has emphasized raw US power.