River Countess aerial exterior shot

(2:44 p.m. EDT) -- Luxury river cruise line Uniworld has announced plans for four new river cruise ships to launch over the next three years.

In 2020 the line will replace its existing ships on the Nile in Egypt, the Douro in Portugal and the Mekong in Vietnam, followed up with a fourth ship, based on the Volga in Russia in 2021.

Uniworld has also revealed it will completely refurbish the 130-passenger Venice-based River Countess, rename it and move it off of the River Po. According to Uniworld CEO Ellen Bettridge, passenger demand has indicated people want to spend more time in the Venice Lagoon, rather than along the River Po.

Uniworld did not give any further details on the new ships, though Bettridge did confirm all the ships would carry fewer passengers than the fleet's existing vessels.

"This is all about elevating our brand, it's not about increasing capacity," she said at a press conference announcing the new ships. "The feedback we get is people want more suites, they want more dining choices, and these new ships will reflect that.

"We are realistic, we have not put two ships in these destinations because we know there is a limit on the number of luxury passengers."

Uniworld currently charters ships in three of these destinations: Queen Isabel on the Douro; Mekong Navigator; and in Russia, River Victoria.

The four new ships will be commissioned, designed to exact specifications by Uniworld.

"The Nile is coming back in a big way," Bettridge added. "We've seen a very nice uptick in bookings. Going forward, we've seen a 45 percent increase in demand to the Nile."

She said the demand was being driven primarily from the U.S., but also by the U.K. and Australia, Uniworld's three biggest markets.

Uniworld's experience reflects that of the wider river sector, with demand gradually increasing for the Nile -- Viking launched Viking Ra this year -- after a slow few years due to terrorism.

--By Kerry Spencer, Editor and Adam Coulter, Managing Editor, UK