The short-listed bidders include Italy's Enel, Germany's E.ON and RWE, Czech CEZ, and Belgium's Electrabel, owned by French utility Suez.
NEK, which will have a controlling stake in the 4.0 billion euro ($5.88 billion) plant, asked for better offers in December, saying it expected improvements in the way the investors saw their role in the future management of Belene.
"All five bidders submitted improved offers," a NEK spokeswoman said. She declined to give further information.
NEK said last month it hoped a strategic partner for the 2,000 megawatt plant at the Danube river town of Belene, to be picked in the first half of the year.
The company had earlier said it planned to wrap up the talks by the end of 2007, but then said negotiations proved hard and more time was necessary.
NEK has contracted Russia's Atomstroyexport, controlled by gas giant Gazprom, along with French Areva and Germany's Siemens to build the plant. The reactors are expected to come online after 2014.
Belene aims to compensate for the closure of two 440-megawatt reactors in 2006 and restore Bulgaria's position as a leading power exporter in southeastern Europe.
NEK has yet to find financing for the plant. It has opened a tender to contract a lead manager to arrange the financing.