Ukraine on April 26 marked 29 years since the Chornobyl nuclear disaster.
The explosion at reactor number four on April 26, 1986, sent a cloud of radiation over large parts of Europe in what was the worst civilian nuclear accident ever.
Ukrainian President Petro Poroshenko laid a wreath at a monument to the nuclear disaster's victims at the crippled plant.
Hundreds of people placed flowers and lit candles at another monument in nearby Slavutych where Chornobyl workers were moved after the accident.
In a statement, a spokesman for UN Secretary General Ban Ki-moon said "We stand in solidarity with the millions who have been traumatized by lingering fears about their health and livelihoods."
The human toll of the disaster is still disputed.
United Nations experts officially recognized 31 deaths among plant workers and firefighters directly linked to the blast.
But environmental group Greenpeace has suggested there would be around 100,000 additional cancer deaths linked to the disaster.
At the time of the accident, Soviet authorities sent tens of thousands of people to put out the fire and clear up the site.
A encasing, called the sarcophagus, was hastily built around the damaged reactor to contain the radiation.
Years ago, however, cracks appeared in that concrete structure, prompting calls to replace it.
On April 26, Poroshenko inspected ongoing work on a new 20,000-ton steel cover -- a project estimated to cost more than two billion euros ($2.2 billion).
It is being financed by international donations managed by the European Bank for Reconstruction and Development (EBRD).
The structure will contain technology to decontaminate the area once the steel layer is in place.
The work is being done by Novarka, a joint venture by French companies Vinci and Bouygues.
Poroshenko said the new cover would "protect forever" against radiation from the site.
The work had been scheduled for completion by the end of this year but the EBRD said last year technical problems would delay it until late 2017.
With reporting by AFP