Even though it happened more than 20 years ago, the world's worst nuclear disaster is still a serious hazard.

President Viktor Yanukovych Monday warned that Ukraine's Chernobyl nuclear plant remains an urgent threat due to lagging safety measures, on the 24th anniversary of the world's worst nuclear disaster.

The plant's fourth nuclear reactor still presents an active danger after work to replace an aging sarcophagus around the facility was delayed due to a shortage of funds last year, Yanukovych said according to a statement.

The death toll from the Chernobyl disaster is bitterly disputed, with a United Nations toll from 2005 setting it at just 4,000, but non-governmental groups suggesting the true toll could reach tens or even hundreds of thousands.

According to Ukrainian official figures, more than 25,000 people known as "liquidators" from then-Soviet Ukraine, Russia and Belarus have died since taking part in the bid to limit radioactive fallout after the catastrophe.