President Biden is facing growing pressure from Congress as his administration wrestles with whether to use U.S. sanctions powers against Nord Stream 2, a Russian natural-gas pipeline project to Europe. Mr. Biden opposes the pipeline, which would circumvent supply lines that run through Ukraine to transport Russian natural gas to U.S. ally Germany, undermining Ukraine and strengthening Russia’s grip on the European energy market. The project, however, was conspicuously left out of a series of sanctions and other punitive measures against Moscow announced Thursday.

The Biden administration has prepared sanctions packages against the project, which are under consideration and include some of the most aggressive approaches yet to kill the pipeline, but hasn’t decided to impose them, according to current and former State Department officials.

These include measures to target Nord Stream 2 AG, the Swiss-registered Russian firm behind the project, and its chief executive officer, Matthias Warnig, an ally of Russian President Vladimir Putin, along with other entities and ships involved in the project, the officials said.

Nord Stream 2 AG declined to comment on the possible sanctions against the company and Mr. Warnig. “The commissioning of Nord Stream 2 is in the interest of Europe’s energy security, European consumers, and EU economic competitiveness,” a Nord Stream 2 AG spokeswoman said.

The measures detailed by the Biden administration on Thursday targeted Russia’s election interference, the SolarWinds cyberattack and other malign activity. However, the Nord Stream project presents a more difficult question, given that Germany wants the pipeline to be completed and that Mr. Biden is working to tighten ties with allies.

“Nord Stream 2 is a complicated issue affecting our allies in Europe,” Mr. Biden said in comments about the new Russian sanctions on Thursday. “I’ve been opposed to Nord Stream 2 for a long time…But that still is an issue that is in play.”

The administration’s deliberations have drawn sharp criticism from Republican lawmakers as well as from Democrats, who say that the White House has been putting off a tough decision to counter what they say are Mr. Putin’s designs on the European energy market and political sphere to avoid upsetting relations with Berlin.

“The lack of Nord Stream 2 sanctions today makes it even more obvious the administration, for unknown reasons, is foot-dragging on this issue,” said Sen. James Risch of Idaho, the senior Republican on the Senate Foreign Relations Committee.

Last month, 40 Republican senators signed a letter urging Mr. Biden to level sanctions. Sens. Jeanne Shaheen (D., N.H.) and Bob Menendez (D., N.J.) did the same in a separate letter.

The $11 billion project will join the first Nord Stream pipeline in allowing the Russian state-owned company Gazprom to pipe gas directly to Germany and effectively abandon the Ukrainian gas transport system that has for decades handled the bulk of deliveries from Russia to Europe. Begun in 2015, it is now more than 95% complete.

In comments to French newspaper Le Figaro Friday, Ukrainian President Volodymyr Zelensky called the project “a matter of war.”