Japanese Prime Minister Fumio Kishida on Sunday criticized Beijing for stepping up actions that infringe on Japan's sovereignty in the East China Sea and warned Russia against using nuclear weapons during its war on Ukraine at an annual summit of the Association of Southeast Asian Nations and its partners.

The East Asia Summit, also attended by U.S. President Joe Biden, Chinese Premier Li Keqiang and Russian Foreign Minister Sergey Lavrov, took place amid tensions over Beijing's assertiveness in regional waters and pressure on Taiwan, while North Korea continues with a barrage of ballistic missile tests.

Kishida was blunt when expressing his concerns about China, saying that Beijing's sovereignty-violating activities "had been continuing and intensifying in the East China Sea," according to the Japanese Foreign Ministry.

The Japanese prime minister was apparently referring to repeated incursions into waters around the Senkaku Islands, a group of East China Sea islets controlled by Tokyo but claimed by Beijing.

He also said peace and stability around Taiwan is an important issue that "directly impacts" regional security, citing how China increased its military activities in reaction to U.S. House of Representatives Speaker Nancy Pelosi's visit to the self-ruled democratic island in August.

During military drills held around Taiwan, China fired ballistic missiles, with five of them falling into Japan's exclusive economic zone in the East China Sea for the first time. Beijing views Taiwan as its own territory.

At the 18-member summit in Cambodia, Biden emphasized the importance of freedom of navigation in the East and South China seas while vowing his country's commitment to the Indo-Pacific.

Ahead of a planned meeting in person with Chinese President Xi Jinping on Monday in Indonesia, Biden commented on U.S.-China relations, underscoring that the United States will "compete vigorously" with China and speak out against Beijing's alleged human rights abuses, according to a White House press release.

It also said Biden hopes to keep the lines of communication open between the world's two largest economies so that competition does not veer into conflict, reaffirming the importance of maintaining peace and stability across the Taiwan Strait.

Biden is on his first trip to Southeast Asia since taking office, with last year's ASEAN-related summit events held virtually amid the coronavirus pandemic.

He also became the first U.S. president in about six years to take part in the East Asia Summit in person, in contrast to his predecessor Donald Trump, who only attended a U.S.-ASEAN summit once, in Manila in 2017, and has never attended full East Asia Summit meetings during his tenure through January 2021.

Trump's absence had sparked concern about a lack of U.S. engagement in the Indo-Pacific at a time when China's clout is growing in the region.

The United States and China are jockeying for influence in fast-growing Southeast Asia, a strategically important region that straddles key sea lanes, including the South China Sea.

ASEAN groups Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Vietnam. The East Asia Summit comprises ASEAN plus Australia, China, India, Japan, New Zealand, Russia, South Korea and the United States.

Kishida and Biden also condemned Russia's war in Ukraine, which has continued since February despite international criticism and sanctions.

Kishida stressed that Russia's threats to use nuclear weapons are "absolutely unacceptable" and called on the international community to send a clear message to prevent the use of such destructive arms, according to the Japanese Foreign Ministry.

The use of nuclear weapons "would be an act of hostility against humanity after 77 years of nonuse of nuclear weapons," he said, speaking as the leader of the only country that has suffered nuclear attacks in war.

Signaling the deep rift among the members, Lavrov said a joint statement on the results of the East Asia Summit was not adopted because the American side and its partners insisted on an "unacceptable assessment of the situation in and around Ukraine," Russia's Tass news agency reported.