Under the finance plans, the European Investment Bank (EIB) would contribute €2bn, the European Bank for Reconstruction and Development (EBRD) €1.2bn and the International Finance Corporation (IFC) - a lending arm of the World Bank - €800 million.
The appraisal mandate signed by the banks and Nabucco shareholders is a step forward for the €7.9bn pipeline, which is designed to bring gas from Central Asia to Europe. The pipeline would have the capacity to transport 31 billion cubic metres of gas per year from the Caspian Sea and Middle East to Austria via Turkey and Eastern Europe.
While any future funding would be subject to the results of the appraisal, the banks gave assurances that they would not embark on it unless they believed the money could be committed at the end.
"The fact that we're willing to identify at this point financial potential of up to four billion euros from the three institutions together can be interpreted as a very serious remark," Thomas Barrett, director at the EIB, told journalists in Brussels.
The pipeline is meant to diversify Europe's natural gas supplies and routes to lessen dependence on Russia.
Reinhard Mitschek, managing director of Nabucco Gas Pipeline International GmbH, said he was not worried about the pipeline being able to get a reliable gas supply from countries like Azerbaijan, Turkmenistan and Iraq, despite political tensions.
"What we see is that Azerbaijan is preparing a tender process for gas supply to Europe. Iraq, irrespective of political considerations, is preparing gas supplies for Europe," he said. It is therefore "an absolute certainty" that they will dedicate gas supplies to Europe, he added, arguing that the only question is who will commit gas first.
In an attempt to alleviate concerns over the environmental impact of the 3,300-kilometre pipeline, the banks stressed that the appraisal would ensure financing to the highest environmental and social standards.
NGO group CEE Bankwatch Network pointed out that the pipeline would cross protected Natura 2000 sites at 12 points in Hungary alone.