Speaking to reporters in Berlin, Oudkirk said the project could sanctioned under a bill passed in August 2017.
Any companies involved were in "an elevated position of sanctions risk", she said.
Mr Johnson responded: "We will of course consider all possibilities once we exit the European Union and take back control of our sanctions policy".
During the meeting with German Chancellor Angela Merkel on April 27, Trump offered to refuse from the support of the gas project in the exchange for new trade agreements between the United States and the EU. The two are expected to discuss the pipeline project as well as the US decision to withdraw from the Iran nuclear deal.
"Germany hooks up a pipeline into Russian Federation, where Germany is going to be paying billions of dollars for energy into Russian Federation". It is due to go online at the end of next year.
"The new project would permit new technologies to be placed along the pipeline route, and that is a threat", she said.More news: In DR Congo identified 11 new cases of infection with Ebola
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She noted the USA has long-opposed projects that would make Europe more dependent on Russian energy sources.
Note that the US and the countries of Eastern Europe are opposed to the construction of the Nord stream - 2, and believe that its implementation will lead to an actual degradation of the Ukrainian gas transportation system and the strengthening of the Russian Federation in the gas market of the EU.
"This pipeline can't be used to blackmail or negatively affect any country", he said.
As previously reported "FACTS", the President of Ukraine Petro Poroshenko warned Germany from participating in the construction of the "Nord stream-2", proposing as an alternative to the modernization of the transit pipeline passing through the territory of Ukraine.
The US believes that Moscow will be able to exert greater influence over Berlin as well as cut about $1 billion a year in transit fees paid to Ukraine for piping natural gas to Europe, and that dependence on the pipeline will weaken the impact of western sanctions on Moscow.
Altmaier has said he is optimistic that a "substantial" amount of gas will flow through Ukraine in the future.