A virtual meeting of the four-country grouping was held at a time of increased concern about Taiwan, a self-ruled island claimed by China, which has stepped up its alert level, wary of China taking advantage of distracted world powers to move against it.

"We've agreed that unilateral changes to the status quo with force like this should not be allowed in the Indo-Pacific region," Kishida said, referring to Russia's invasion of Ukraine.

"We've also agreed this development makes it even more important to work toward realising a free and open Indo-Pacific," Kishida told reporters after the meeting with US President Joe Biden, Australian Prime Minister Scott Morrison and Indian Prime Minister Narendra Modi.

A joint statement issued after the call by the White House said the Quad leaders met to "reaffirm their commitment to a free and open Indo-Pacific, in which the sovereignty and territorial integrity of all states is respected and countries are free from military, economic, and political coercion".

It said they also "reaffirmed their dedication to the Quad as a mechanism to promote regional stability and prosperity".

The statement made no specific mention of Taiwan, but said the leaders discussed the conflict and humanitarian crisis in Ukraine.

"They agreed to stand up a new humanitarian assistance and disaster relief mechanism which will enable the Quad to meet future humanitarian challenges in the Indo-Pacific and provide a channel for communication as they each address and respond to the crisis in Ukraine," it said.

Biden tweeted that the meeting with the Quad leaders covered "our commitment to sovereignty and territorial integrity around the world, including in the Indo-Pacific".

Kurt Campbell, the White House coordinator for Indo-Pacific, said on Monday the United States would keep its focus on the Indo-Pacific despite the Ukraine crisis, although this would be difficult and expensive.

He said the US has been deeply engaged in two theatres simultaneously before, including during World War II and the Cold War.

The United States sees the Quad and its growing relations with India as essential to its efforts to push back against China in the Pacific but it is in a delicate balancing act with India, given the latter's long-standing ties with Russia.

Of the four Quad countries, only India has not condemned Russia's invasion of Ukraine.

Russia is the main supplier of arms to the Indian military and India faces the possibility of US sanctions for its purchase of the Russian S-400 air defence system.

Analysts say any moves by Russia hawks in Biden's administration to impose sanctions on India for working with Russia could backfire and set back co-operation in the Quad.

Campbell said on Monday that the US remained "bullish" about its relationship with India.

"We have a deep dialogue with them on issues underway now," he told a Washington DC think tank.

"We understand ... India's historic, long-standing relationship with Russia, but at the same time, ultimately, we believe that India will be moving in our direction."

India's foreign ministry said before Thursday's meeting that it would follow a September summit of the Quad leaders in Washington DC and they would "exchange views and assessments about important developments in the Indo-Pacific".

It was not immediately clear on whose request the meeting was called.

None of the Quad countries had flagged it earlier.

Quad foreign ministers met in Australia early last month and pledged to deepen co-operation to ensure the region was free from "coercion," a veiled reference to China's economic and military activities, and their leaders are set to hold a summit in Japan in May.

China has denounced the Quad as a Cold War construct and a clique "targeting other countries".

The White House said on Friday that the Quad leaders agreed to meet in person in Tokyo in the coming months.

Australian Associated Press