Sullivan said Biden would not make demands on China but rather give Xi "his perspective".

This is that "North Korea represents a threat not just to the United States, not just to (South Korea) and Japan but to peace and stability across the entire region."

Whether China wants to increase pressure on North Korea is "of course up to them", Sullivan said.

However, with North Korea rapidly ramping up its missile capacities, "the operational situation is more acute in the current moment", Sullivan said.

Japanese prime minister Fumio Kishida added his voice to calls for concerted international action to halt Pyongyang's missile programme during talks with ASEAN, China and South Korea.

Tokyo and Seoul have been increasingly alarmed by the North Korean testing blitz, which included an intercontinental ballistic missile.

Biden and Xi, the leaders of the world's two biggest economies, have spoken by phone multiple times since Biden became president in January 2021.

But the Covid-19 pandemic and Xi's subsequent aversion to foreign travel have prevented them from meeting in person.

Regional rivalry

The pair is not short of topics to discuss, with Washington and Beijing at loggerheads over issues ranging from trade to human rights in China's Xinjiang region and the status of the self-ruled island of Taiwan.

UN chief Antonio Guterres has urged the two sides to work together, warning Friday of "a growing risk that the global economy will be divided into two parts, led by the two biggest economies -- the United States and China".

Before the G20, Biden will push the US's commitment to Southeast Asia in meetings with leaders from the Association of Southeast Asian Nations (ASEAN), seeking to counter Beijing's influence in the region.

China has been flexing its muscles -- through trade, diplomacy and military clout -- in recent years in a region it sees as its strategic backyard.

Biden flew into Phnom Penh with an agenda emphasising his administration's policy of "elevating" the US presence in the region as a guarantor of stability, Sullivan said.

Biden will argue for "the need for freedom of navigation for lawful, unimpeded commerce, and for ensuring that the United States is playing a constructive role in maintaining peace and stability in the region".

Xi emerges, Putin absent

Biden and Xi both go into the G20 buoyed by recent domestic political success: Biden's party having earned surprisingly strong midterm results and Xi having secured a landmark third term as China's leader.

At last month's Communist Party Congress, where he was anointed as chief again, Xi warned of a challenging geopolitical climate without mentioning the United States by name, as he wove a narrative of China's "inevitable" triumph over adversity.

The G20 summit will be the latest step in a diplomatic re-emergence for Xi after the pandemic -- it comes less than a fortnight after he hosted German Chancellor Olaf Scholz in Beijing.

As well as Biden, Xi will also meet French president Emmanuel Macron before heading to Bangkok later in the week for the APEC summit.

Notably absent from the summit will be Russian president Vladimir Putin, who has been shunned by the West over his invasion of Ukraine, and who is instead sending foreign minister Sergei Lavrov.

Lavrov will press Moscow's view that the United States is "destabilising" the Asia-Pacific region with a confrontational approach, the Russian TASS news agency reported.

Ukrainian president Volodymyr Zelensky is expected to attend the G20 virtually, after his request to address the ASEAN gathering was turned down.