Nabucco is up against the smaller Interconnector Turkey-Greece and the Trans Adriatic Pipeline projects included as part of the so-called Southern Corridor. Nabucco is the most ambitious and there's not enough natural gas available from Azerbaijan to fill its planned yearly capacity.
Nabucco authorities said it would look to feeder lines, notably from Iraq, to provide additional capacity to the planned pipeline.
Richard Morningstar, U.S. special envoy for Eurasian energy, told the Interfax news agency that, given political turmoil in Iraq, it could be a number of years before natural gas exports are possible.
"But there is no way to predict when that is going to happen," he said.
He maintained, however, that Washington was a key supporter of the ambitious pipeline. Europe wants to advance the Southern Corridor to break Russia's grip on the regional energy sector.
Morningstar said Nabucco would be "preferable" but it must be commercially viable.
"At this point, we support any pipeline through the Southern Corridor that is commercially viable and meets certain conditions such as providing diversification to those countries that are reliant on a single source of supply and guaranteeing that the pipeline could be expanded to cope with additional gas that will come online in the future," he said.