A major drought along the lower Danube has highlighted the river's reduced ability to buffer extreme weather events and has hampered navigation, environmental group WWF said on Tuesday.

The Danube is currently seeing its lowest levels since 2003, with bigger ships often being blocked by multiple sand bars, according to Bulgaria's Danube exploration agency.

Human intervention such as gravel extraction, dredging and dams has contributed to the gradual loss of wetlands that can soak up water during rainy periods and release it slowly during drier times, WWF said.

A WWF bird-watching expedition during the summer found as a result a drastic drop in bird populations along the lower stretch of the river in Bulgaria and Romania.

"The current dry conditions highlight the need to minimise the impact of structural interventions (...) as they reduce the resilience of water ecosystems," WWF Danube-Carpathian programme director Andreas Beckmann said.

He raised the need to rethink plans for new infrastructure works to eliminate rapids and improve navigation as well as the planned construction of new hydropower stations on the river.

According to Beckmann, these may affect the ecological status of freshwater ecosystems along the whole 1,000-kilometre (620-mile) lower stretch of the Danube.

Besides, "the feasibility of hydropower and navigation projects relies on predictable water levels while climate change is expected to lower predictability," another WWF expert, Irene Lucius, warned.

Low water levels already blocked around 100 ships Tuesday on the upstream Serbian stretch of the Danube, local media said. Traffic is not expected to resume over the coming days as meteorological services forecast no rain.

The 2,860-kilometre Danube, Europe's second longest river, originates in Germany and passes through eight central and eastern European countries before flowing into the Black Sea.