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 construct a new shelter to permanently secure the stricken reactor.

An explosion during testing in the early hours of April 26, 1986, 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.

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 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).

The completion of the new shelter is expected in October 2015.

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 United Nations 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.