* Overnight missile attack struck town in Kherson region

* Ukrainian forces said they hit ammunition depot

* Russian-installed authorities say people still trapped (Recasts headline and lead, changes dateline)

LONDON, July 12 (Reuters) - At least seven people were killed and 70 wounded in an overnight missile attack by Ukrainian forces on the Russian-held town of Nova Kakhovka in Ukraine's southern Kherson region, the Russian-installed administration there said on Tuesday.

Unverified footage on social media showed smoke and sparks, followed by an immense fireball erupting into the night sky. Pictures showed rubble strewn across streets and scorched buildings.

Officials from the Russian-installed administration said that Ukraine struck with U.S. supplied HIMARS missiles that destroyed warehouses containing saltpeter, a chemical compound which can be used to make fertilizer or gunpowder, resulting in a large explosion.

Ukrainian officials said their forces had destroyed an ammunition depot in Nova Kakhovka. Serhiy Bratchuk, Odesa administration spokesperson, wrote on his Telegram channel that Nova Kakhovka was now "minus" its ammunition warehouse.

"There are already seven dead for sure," TASS quoted Vladimir Leontyev, head of the Russia-installed Kakhovka District military-civilian administration.

"There are still many people under the rubble. The injured are being taken to the hospital, but many people are blocked in their apartments and houses," Leontyev added.

The city's hydroelectric power plant was not damaged, RIA cited Kirill Stremousov, deputy head of the Russian-controlled administration in Kherson as saying.

Reuters could not immediately verify battlefield reports.

Ukraine has said it expects a fresh assault by Russian ground forces, following widespread shelling which killed more than 30 people.

Since Russia sent troops into Ukraine on Feb. 24 in the biggest ground invasion in Europe since World War Two, thousands have been killed, millions displaced and whole swathes of some Ukrainian cities turned into wastelands.

Both sides have accused each other of targeting civilians though both sides deny they do.

Ukraine and its Western backers say that President Vladimir Putin has no justification for what they say is an imperial-style land grab against a country whose borders Moscow recognised as the Soviet Union collapsed.

Putin says the "special military operation" in Ukraine is necessary because Moscow had to defend Russian-speaking people against persecution which he says the West has ignored.