China is under intense diplomatic pressure from the US and its European allies to pull its lifeline from an isolated Russia but, three weeks after the invasion of Ukraine, Beijing has shown few signs of abandoning its friends in the Kremlin.

Washington has driven Russia to the cusp of default since Vladimir Putin's invasion of neighbouring Ukraine, strangling its economy with sanctions and pulling the country from the global payments system.

Isolated, bleeding money and with its currency in freefall, Russia has grasped for the friendship of its giant southwestern ally -- China.

And China appears to have reciprocated, despite the risks to its both reputation and its economic interests as the US-led sanctions unwind.

A senior US official on Monday voiced "deep concerns about China's alignment with Russia" following talks in Rome between US National Security Advisor Jake Sullivan and Yang Jiechi, the Chinese Communist Party's chief diplomat.

The discussions aimed to test the depth of China's commitment to Russia as it struggles to vanquish Ukraine, with images of bomb-scarred buildings and refugees fleeing in their hundreds of thousands horrifying the world.

Since the war erupted, China has refused to condemn Putin's actions - or even describe the invasion as a "war".

Instead, as recently as last week Beijing called the partnership between the two "rock-solid".

Moscow has dangled the prospect of cheap oil and gas to its energy-hungry neighbour, while the west vows to unhook itself from Russian hydrocarbons.

Analysts say President Xi Jinping's challenge is to pick a pathway through the sudden chaos unleashed on the international order by Putin's actions -- and ensure China comes out ahead of its rival, the US.

"China is in it for self-interest, period. A weaker Russia is probably a Russia where you can do more that serves your interests, since you have more leverage," said Alexander Gabuev, senior fellow at the Carnegie Center Moscow.

In its public diplomacy, Beijing seeks to position itself as a neutral player, calling for peace negotiations and condemning NATO for providing military assistance to Ukraine.

Isolated, bleeding money and with its currency in freefall, Russia has grasped for the friendship of giant southwestern ally -- China
Isolated, bleeding money and with its currency in freefall, Russia has grasped for the friendship of giant southwestern ally -- China SPUTNIK via AFP / Alexei Druzhinin

But US media reports, citing intelligence briefings, suggest Beijing may be ready to pull the Russian economy back from the brink of default and even meet Moscow's entreaties for weapons and logistical support for its struggling troops in Ukraine.

Alarmed by the potential emergence of a new authoritarian world order, EU officials including its foreign policy chief Josep Borrell have lobbied China over recent weeks to exert its "considerable influence" on Russia to end the war.

Those calls appear to have fallen flat.

"Why would China even consider not to support Russia, or even worse, undermine (its) alliance with Russia?" said Alexander Korolev, Senior Lecturer in Politics and International Relations at the University of New South Wales in Sydney.

"I think China is unlikely to change its position and this will indicate a deeper divide of global geopolitics."

Indications of China's next steps are being scoured out from the oblique comments of its diplomats.

On Tuesday state media carried a message from Wang Yi, Beijing's foreign minister, stating "China is not a party to the crisis, still less wants to be affected by sanctions."

Already global commodity prices have surged -- especially of wheat and corn, which flows from the Russia and Ukrainian breadbasket to China.

Meanwhile Chinese state media has been amplifying Russian propaganda messages.

Those include allegations that the Ukrainian military is using civilians as "human shields" and a conspiracy theory that the United States is developing biological weapons in the former Soviet state.

But for all the chumminess between Xi and Putin, the two countries have a long history of mistrust which is yet to be wiped away.

"China's stance is not so much pro-Russia as anti-US," explains Alexander Gabuev, a China-Russia expert at the Carnegie Center Moscow.

And the unexpected war, which came three weeks after Putin visited Xi in Beijing, may yet challenge a friendship described as having "no limits" following that tour.

"The more protracted and brutal the conflict, the more difficult the situation for China will be," said Ni Lexiong, a professor at the Shanghai Institute of National Defence Strategy.