HIROSHIMA, Japan -- Leaders of the world's most powerful democracies gathered Thursday for Group of Seven meetings in Hiroshima, with Russia's invasion of Ukraine high on the agenda for a summit convened in the shadow of the world’s first atomic bomb attack.

The G7 nations, which officials said have reached new levels of cooperation more than a year into Russia's brutal war, were set to unveil a new round of sanctions against Moscow when the summit officially opens on Friday, as well as announce that they would redouble their efforts to enforce existing sanctions meant to stifle Russia's war effort and punish those behind it, a U.S. official said.

The official, speaking on the condition of anonymity to preview the announcement, said the U.S. component of the actions would blacklist about 70 Russian and third-country entities involved in Russia's defense production, and sanction more than 300 individuals, entities, aircraft and vessels.

The official added that the other nations in the group would undertake similar steps to further isolate Russia and to undermine its ability to wage war in Ukraine. Details were to come out over the course of the weekend summit.

Japanese Prime Minister Fumio Kishida, who is hosting the summit in his hometown, opened the global diplomacy with a sitdown with U.S. President Joe Biden after Biden's arrival at a nearby military base. Kishida also held talks with British Prime Minister Rishi Sunak before the three-day gathering of leaders opens.

The Japan-U.S. alliance is the “very foundation of peace and security in the Indo-Pacific region,” Kishida told Biden in opening remarks.

“We very much welcome that the cooperation has evolved in leaps and bounds,” he said.

Biden, who greeted U.S. and Japanese troops at nearby Marine Corps Air Station Iwakuni before his meeting with Kishida, said: “When our countries stand together, we stand stronger, and I believe the whole world is safer when we do."

As G7 attendees made their way to Hiroshima, Moscow unleashed yet another aerial attack on the Ukrainian capital. Loud explosions thundered through Kyiv during the early hours, marking the ninth time this month that Russian air raids have targeted the city after weeks of relative quiet.

“The crisis in Ukraine: I’m sure that’s what the conversation is going to start with,” said Matthew P. Goodman, senior vice president for economics at the Center for Strategic and International Studies.

Speaking to reporters aboard Air Force One, Jake Sullivan, the White House national security adviser, said there will be “discussions about the battlefield” in Ukraine and on the "state of play on sanctions and the steps that the G7 will collectively commit to on enforcement in particular.”

Russia is now the most-sanctioned country in the world, but there are questions about the effectiveness of the financial penalties despite their breadth.

The U.S., for example, has frozen Russian Central Bank funds, restricted banks’ access to SWIFT -- the dominant system for global financial transactions -- and sanctioned thousands of Russian firms, government officials, oligarchs and their families.

The Group of Seven nations collectively imposed a $60 per-barrel price cap on Russian oil and diesel last year, which the U.S. Treasury Department on Thursday defended in a new progress report, stating that the cap has been successful in suppressing Russian oil revenues. Treasury cites Russian Ministry of Finance data showing that the Kremlin’s oil revenues from January to March this year were more than 40% lower than last year.

The economic impact of sanctions depends largely on the extent to which a targeted country is able to circumvent them, according to a recent Congressional Research Service repor t. So for the past month, U.S. Treasury officials have traveled across Europe and Central Asia to press countries that still do business with the Kremlin to cut their financial ties.

G7 leaders and invited guests from several other counties are also expected to discuss how to deal with China's growing assertiveness and military buildup as concerns rise that it could could try to seize Taiwan by force, sparking a wider conflict. China claims the self-governing island as its own and its ships and warplanes regularly patrol near it.

Security was tight in Hiroshima, with thousands of police deployed throughout the city. A small group of protesters was considerably outnumbered by police as they gathered Wednesday evening beside the ruins of the Atomic Peace Dome memorial, holding signs including one which read “No G7 Imperialist Summit!”

In a bit of dueling diplomacy, Chinese President Xi Jinping is hosting the leaders of the Central Asian countries of Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan and Uzbekistan for a two-day summit in the Chinese city of Xi’an starting Thursday.

During the meeting in Hiroshima, Kishida hopes to highlight the risks of nuclear proliferation. The leaders on Friday are scheduled to visit a memorial park that commemorates the 1945 atomic bombing by the U.S. that destroyed the city and killed 140,000 people.

North Korea’s nuclear program and a spate of recent missile tests have crystalized fears of a potential attack. So have Russia’s threats to use nuclear weapons in Ukraine. China, meanwhile, is rapidly expanding its nuclear arsenal.

The leaders are due to discuss efforts to strengthen the global economy and address rising prices that are squeezing families and government budgets around the world, particularly in developing countries in Africa, Asia and Latin America.

The debate over raising the debt limit in the U.S., the world's largest economy, has threatened to overshadow the G7 talks. Biden plans to hurry back to Washington after the summit for debt negotiations, scrapping planned meetings in Papua New Guinea and Australia.

The British prime minister arrived in Japan earlier Thursday and paid a visit to the JS Izumo, a ship that can carry helicopters and fighter jets able to take off and land vertically.

During their bilateral meeting Thursday, Sunak and Kishida announced a series of agreements on issues including defense; trade and investment; technology, and climate change, Sunak's office said.

The G7 includes Japan, the United States, the United Kingdom, France, Germany, Canada and Italy, as well as the European Union.

A host of other countries have been invited to take part. The G7 hopes to strengthen its members' ties with countries outside the world's richest industrialized nations, while shoring up support for efforts like isolating Russia.

Leaders from Australia, Brazil, India, Indonesia and South Korea are among those participating as guests. Ukrainian President Volodymyr Zelenskyy is expected to join by video link.