TOKYO, May 9 (Reuters) - The leaders of the Group of Seven (G7) rich nations must condemn any threat to use nuclear weapons and vow "decisive action" against such a move when they hold a summit next week in the city of Hiroshima, Ukraine's envoy to Japan said.

Nuclear tension has surged since the start of the conflict with Ukraine as President Vladimir Putin has repeatedly warned that Russia is ready to use its nuclear arsenal, if necessary, to defend its "territorial integrity."

Sergiy Korsunsky also told Reuters that conditions around the Zaporizhia nuclear power plant had deteriorated rapidly, a situation the head of the U.N.'s nuclear power watchdog warned at the weekend had become "potentially dangerous". read more

"It should be a very clear statement from specifically those nuclear powers among the G7 that the use of nuclear weapons or nuclear terrorism will not be tolerated and will be met with almost decisive actions from major powers," he said.

The envoy's comments came as Russia launched drone, missile and air strikes on the Ukrainian capital, Kyiv, and other cities in the run-up to its cherished Victory Day holiday on May 9 that celebrates the defeat of Nazi Germany.

Ukraine's President Volodomyr Zelenskiy will use video link to attend the G7 gathering from May 19 to 21 in Hiroshima, which became the world's first city to suffer an atomic bombing in 1945, adding significance to his country's message.

"It's most important that the summit when we have a real threat of nuclear terrorism, that summit will be in Hiroshima," Korsunsky added.

He called for G7 talks on nuclear security and the global architecture, as both the International Atomic Energy Association (IAEA) and the U.N. Security Council lacked power.

"Maybe additional decisions should be taken" about a new system, he added.

"We believe that among the G7 there must be consensus if we want a bigger system of international security to protect any country in the world. We're not talking about just Ukraine."

Ukraine's other big hope is that the summit will discuss efforts for reconstruction - in which Korsunsky hopes Japan can play a key role, in light of its experience rebuilding from disasters - particularly using frozen Russian assets to do so.

In February, the European Union said it had formed an ad hoc group to see about using frozen Russian funds to rebuild Ukraine.

Last week, Belgian Prime Minister Alexander De Croo said his government was examining how Russian assets could be used to help Ukraine's war effort.