Prime Minister Yoshihide Suga agreed Thursday with new U.S. President Joe Biden to strengthen the Japan-U.S. alliance and work closely to realize a free and open Indo-Pacific region, in their first phone talks since the American leader was inaugurated last week.

Biden reiterated the United States' "unwavering commitment" to protecting the Senkaku Islands, a group of uninhabited islets administered by Japan but claimed by China, according to Japan's Foreign Ministry and the White House.

Suga is keen to build close ties with Biden amid mounting security challenges including China's military buildup and North Korea's nuclear and missile development programs.

Japanese Prime Minister Yoshihide Suga speaks to reporters in his official residence in Tokyo after his first phone talks with U.S. President Joe Biden on Jan. 28, 2021 (Kyodo)

"I would like to deepen my personal relationship with President Biden and work to strengthen the Japan-U.S. alliance," Suga told reporters after the roughly 30-minute conversation, adding that he hopes to visit the United States as soon as possible despite the challenges of overcoming the coronavirus pandemic in both countries.

After Biden thanked Suga for agreeing to hold the talks after midnight Tokyo time, the two agreed to refer to each other by their first names, Yoshi and Joe, to demonstrate the closeness of their countries' bilateral relationship.

The leaders' agreement that the Senkakus in the East China Sea are covered by Article 5 of the 1960 Japan-U.S. security treaty comes after the defense chiefs and foreign ministers of both countries confirmed the point in recent phone talks.

China last week passed legislation authorizing coast guard ships, which it frequently sends near the Senkakus as well as into the disputed South China Sea, to use weapons against foreign ships deemed to be involved in illegal activities.

Speaking to the media in the morning, Chief Cabinet Secretary Katsunobu Kato welcomed the United States mentioning the Senkakus issue, saying, "It is very meaningful that (the United States) reiterated the intent to further strengthen the deterrence of the Japan-U.S. alliance under the Biden administration."

Suga and Biden agreed to work with Australia and India, fellow members of the so-called Quad of regional democracies, to step up defense cooperation amid concerns over China's growing assertiveness across the Indo-Pacific region, according to the Foreign Ministry.

The leaders also affirmed cooperation in tackling global issues including the coronavirus pandemic, which has killed more than 2 million people worldwide and upended the economies of both countries, as well as climate change.

Suga welcomed Biden's decisions to return to the 2015 Paris Agreement on curbing carbon emissions and not to withdraw from the World Health Organization, and the U.S. leader invited Suga to participate in a summit on climate change slated for April 22, the Foreign Ministry said.

The two countries share the goal of achieving net zero emissions by 2050, and the Japanese government said it believes the climate change summit in the United States will facilitate international efforts to fight global warming in the run-up to the U.N. Climate Change Conference to be held in November in Britain.

On North Korea, the two agreed on the necessity of complete denuclearization in line with U.N. Security Council resolutions, while Suga called for Washington's support in resolving the issue of the North's abduction of Japanese nationals in the 1970s and 1980s.

Their discussions also covered South Korea, which has been at odds with Japan over issues including wartime labor and "comfort women" forced to work at Japanese military brothels, but government officials did not disclose details of their conversations on the topic.

They said the two did not discuss the Tokyo Olympic and Paralympic Games, which had originally been scheduled for the summer of 2020 but were postponed for one year due to the coronavirus outbreak.

"Of course we need the cooperation of the United States and other countries in order to successfully hold the Olympics," Kato said, but added it was not touched on in the talks as the leaders focused on other issues.

While the central government, the Tokyo metropolitan government and the International Olympic Committee have repeatedly vowed to hold the games, speculation is rife that they may not happen as scheduled as the world continues to battle the pandemic.

Concerns have deepened with a resurgence of virus infections in Tokyo and other parts of the country that has forced the Japanese government to declare a second state of emergency for the areas.

Biden was inaugurated on Jan. 20 after defeating Donald Trump in the November presidential election and has since made a series of calls to world leaders, including British Prime Minister Boris Johnson, German Chancellor Angela Merkel and Russian President Vladimir Putin.