He made the comments during a trip to the Chernobyl exclusion zone, an area spanning a 30-kilometre (20-mile) radius around the nuclear power plant that was evacuated in the aftermath of the accident and has been deemed unsafe for humans to live for thousands of years to come.

"Our task is to do everything possible to bolster security and strengthen safety to avoid and never repeat a similar disaster in the future," Zelensky said in an address to Ukrainians, leading nationwide commemorative events.

In the capital Kiev, religious leaders and elderly veterans in military uniform involved in the fraught clean up efforts immediately after the incident joined a ceremony and placed flowers at a memorial.

"History does not go backwards," Zelensky said in his televised speech, "so Chernobyl today is a common challenge and a joint responsibility for the future and safety of the planet".

The deaths of some 30 people were directly linked to the explosion in the fourth reactor at Chernobyl during a safety test on April 26, 1986.

But thousands more are feared to have died in the years that followed from radiation poisoning across Ukraine as well as its northern neighbour Belarus and Russia to the east.

Ukraine's security service on Monday released a cache of archive documents, confirming that Soviet authorities hushed up at least three accidents at the Chernobyl plant in 1982 and 1984.

The Chernobyl power plant and two others in present-day Russia with the same reactors had been "dangerous" and continued operations could have had "threatening consequences," according to KGB secret service documents dated 1983 that Ukrainian authorities published Monday.

- Exclusion zone 'renaissance' -

The exact number of victims remains a subject of intense debate because the Soviet authorities kept most of the information about the disaster hidden.

Hundreds of thousands of people were evacuated from the area around the disaster site, and the exclusion zone has now become a ghostly uninhabited region.

About 600,000 emergency workers and state employees who became known as "liquidators" were dispatched with little or no protective gear to help clean up the aftermath of the disaster.

A 2005 United Nations report estimated that up to 4,000 people could eventually perish from the invisible poison in Ukraine, Russia and Belarus.

The Greenpeace environmental group issued its own study the following year estimating that 100,000 people had already lost their lives as a result of the accident.

In November 2016, a massive metal dome was erected over the remains of the reactor -- paid for with 2.1 billion euros ($2.5 billion) in international funding -- to stop future leaks and ensure the safety of Europeans for generations.

Ukrainian authorities say the area maybe not be fit for humans for 24,000 years, but the site has been attracting more tourists in recent years and Kiev wants to make it a UNESCO World Heritage Site.

Zelensky in his address Monday repeated his ambition to overhaul the exclusion zone and render it both an attractive tourist destination and also an environmental haven.

"Our goal today is to transform the exclusion zone ... into an area of renaissance."

Culture Minister Oleksandr Tkachenko said Monday that gaining UNESCO status for the Chernobyl zone would aid preservation efforts and allow future generations to "to see everything with their own eyes".

"But Chernobyl is not about entertainment. It is about memory and responsible tourism," he added.

Meanwhile, surging flora and fauna have taken over deserted tower blocks, shops and official buildings, as the area turned into a giant reserve unprecedented in Europe in size.

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