Richard Morningstar, the US envoy, told a news conference in the Azeri capital on 15 November that Nabucco still has Washington's backing but economic concerns - including the potential to scale the pipeline to available gas volumes - should take precedence.
"It's important if Shah Deniz producers and [Azeri state oil company] SOCAR choose a smaller pipeline as the first pipeline," Morningstar said after meeting the Azeri President Ilham Aliyev.
"It will be extendable as more gas becomes available," he added.
Three consortiums are bidding to carry gas to Europe from the second phase of Azerbaijan's giant Shah Deniz field.
They are aiming to secure a new alternative to Russian supplies, seen as prone to political influence and vulnerable to supply interruptions during pricing rows between Russia and transit countries such as Ukraine and Belarus.
Two competing projects - the Trans Adriatic Pipeline (TAP) and Interconnector Turkey-Greece-Italy (ITGI) - would run through Greece and could hit financial obstacles if Greece defaults or exits the eurozone.
The European Union is backing the 31-billion-cubic-metre, 3,000-kilometre Nabucco pipeline, which is designed to tap both Caspian and Middle Eastern suppliers.
But at a cost of €10 billion, Nabucco is "not cheap", EU Energy Commissioner Günther Oettinger said on 4 November.
In the debate surrounding the routes, a darkhorse option has emerged in the form of a pipeline backed by BP, one of SOCAR's partners in Shah Deniz.
The other partner in Shah Deniz is Norway's Statoil, a leader of the TAP consortium.
SOCAR and its partners in Shah Deniz plan to decide on an export route for 10 billion cubic metres of gas per year from Shah Deniz II by the end of this year.
Shah Deniz is believed to hold 1.2 trillion cubic metres of gas, which European companies hope can supply them for decades, reducing their dependence on Russia. Production began in 2006, with the second phase expected in 2017.
Russia is planning its own new southern route to bypass Ukraine, which is already set to see its share of transit volumes fall with the launch last week of the first phase of Nord Stream, an undersea Baltic route that will ultimately carry 55 billion cubic metres per year.
A final investment decision on the South Stream pipeline, which would run along the floor of the Black Sea, was taken earlier this year.
Ukraine, which until the launch of Nord Stream carried 80 % of Russia's transit volumes to Europe, has quarrelled repeatedly over the price of the gas it buys from Russia's Gazprom, leading to supply interruptions to Europe in 2006 and 2009.
The two are seeking a resolution that could put Ukraine's transit pipelines under the joint control of Russia, Ukraine and a European entity, but those talks have yet to bring a result.
The outcome of those talks could influence the need for South Stream and the so-called "southern corridor" options under discussion for Azeri gas.