Ukrainians on Friday lit candles and laid flowers to remember the victims of the world's worst nuclear disaster at Chernobyl 27 years ago, as engineers pressed on with efforts to permanently shield the stricken reactor.

On April 26, 1986, an explosion during testing sent radioactive fallout into the atmosphere that spread across Europe, particularly contaminating Belarus, Ukraine and Russia.

Dozens of people laid flowers and set lit candles in front of portraits at the monument to the Chernobyl victims in the small town of Slavutych, some 50 kilometres from the accident site, where many of the power station's personnel used to live.

At the same time in the capital Kiev, officials and relatives of the victims also held a pre-dawn remembrance ceremony in front of a memorial.

"The memory of the tragedy calls for unity and consolidation of the efforts of the government and society to solve the problems in implementing projects to create an environmentally safe system at Chernobyl," said President Viktor Yanukovych in a statement.

"The countless women, men and children affected by radioactive contamination must never be forgotten," UN spokesman Martin Nesirky said in a statement, urging worldwide "generosity" to the affected regions.

Ukraine last year launched the construction of a permanent shelter to replace the temporary concrete-and-steel edifice that was hastily erected after the disaster and which has since developed cracks.

"A new confinement is our future, this is something that we awaited for many years," Alexander Novikov, deputy technical director for security at the Chernobyl power plant, told reporters on a visit to Chernobyl this week.

The 20,000-tonne arched structure that spans 257 metres, known as the new safe confinement, is designed to last for a century, and will contain hi-tech equipment to carry out safe decontamination work inside the ruined reactor.

The construction of the new structure is expected to cost 990 million euros, while the decontamination work on the site will push the total cost up to 1.5 billion euros ($2 billion).

Completion of the new shelter is expected in October 2015.

The plant's management said it will also soon begin construction of a temporary cover over the section of Chernobyl plant where a part of the roof collapsed this winter under the weight of fallen snow.

Novikov emphasised that the section, which collapsed in February, was not the part of the sarcophagus structure covering the exploded reactor.

"The project work is almost completed and we will start construction of temporary cover to close the hole that appeared," he said.

The general manager of the Chernobyl plant, Igor Gramotkin, added the collapse of the roof section once again underlined the need for the rapid completion of a new arch over the stricken reactor.

Chernobyl is only around 100 kilometres (60 miles) from Kiev and lies close to the borders with Russia and Belarus. The area around the plant is still very contaminated and is designated as a depopulated "exclusion zone".

The Soviet Union ordered thousands of people to take part in the clean-up in Ukraine following the Chernobyl accident, working without adequate protection.

Although only two people were killed in the initial explosions, the UN atomic agency says that 28 rescue workers died of radiation sickness in the first three months after the accident.

According to Ukrainian official figures, more than 25,000 of the cleanup workers, known as "liquidators" from then-Soviet Ukraine, Russia and Belarus have died after the disaster.

However the true scale of the death toll directly attributable to the disaster remains the subject of bitter scientific debate.