Japanese Prime Minister Fumio Kishida and other leaders from the G7 rich nations, which also include the United States, Germany, Britain, France, Italy and Canada, were due to visit the peace memorial in the city levelled by an atomic bomb in World War II.
The leaders are later expected to announce tightened sanctions on Russia and debate strategy on a more than year-long conflict that shows no signs of easing.
The UK will announce a ban on Russian diamonds and imports of metals from Russia including copper, aluminium and nickel in support of Ukraine, its government said in a statement.
Britain is also targeting an additional 86 people and companies from Russian President Vladimir Putin's military-industrial complex, in addition to those involved in the energy, metals and shipping industries.
European Council President Charles Michel said on Friday Europe would restrict sales of Russian diamonds.
Hiroshima's singular place in human history as the first city to have a nuclear weapon used against it made it a sombre platform on a rainy, overcast day as the leaders gathered at its peace memorial park.
Officials were still hashing out the details of their final announcements on Russia as well as debating precise language on China, according to people from four of the nations involved.
Moscow has said it is ready to use its nuclear arsenal to defend its "territorial integrity" if necessary.
Kishida, who represents Hiroshima in Japan's lower house of parliament, said he chose the city for the summit to focus attention on arms control.
"You will hear a powerful statement of unity, strength and commitment in our response to Russia's war of aggression," a senior US administration official said.
"You will see new steps taken to economically isolate Russia and to weaken its ability to wage war."
The US is set to add 70 entities to its export blacklist, and to expand its sanctions authority to 300 entities as well as new sectors of the Russian economy, the official said.
The goal is to close evasion loopholes in countries from Europe to Asia and the Middle East, target goods used by Russia in waging the war, reduce reliance on that country's energy exports and cut Moscow's access to the international financial system.
