A Russian-backed referendum called for this weekend in eastern Ukraine could trigger new sanctions against Moscow, the Obama administration said Tuesday.

“If we have a separatist referendum that is recognized by Russia and results in Russian peacekeepers” moving into eastern Ukraine, “that will be a trigger,” Victoria Nuland, the State Department’s top diplomat for Europe, told a Senate hearing.

The scenario would repeat Russian actions in the Ukrainian region of Crimea. After a March referendum asking whether citizens there would rather be part of Russia, Russian troops appeared and Moscow announced it had annexed the region.

Nuland’s comments came as lawmakers on the Senate Foreign Relations Committee pressed her, along with Pentagon and Treasury Department officials, to clarify President Obama’s pledge last week to impose sanctions against whole sectors of the Russian economy if Russia continued to destabilize Ukraine and interfered with a nationwide election its government has scheduled for May 25.

“Why wouldn’t we just do it now, if we know that’s where [the situation in eastern Ukraine] is headed?” Sen. Marco Rubio (R-Fla.) asked about the sanctions.

Map: Russia and Ukraine are positioning their troops for war.

Nuland said that the administration was watching the situation in the east “intensely” but that U.S. actions would be more effective if they were coordinated with the still-skittish European Union, whose 28 members are not agreed on the sanctions.

“Keeping the cats herded is a challenge for the Europeans,” Nuland said.

In a separate news conference, Secretary of State John F. Kerry called the referendum effort “contrived” and “bogus.”

“We flatly reject this illegal effort to further divide Ukraine,” Kerry said at a news conference with visiting EU foreign policy chief Catherine Ashton.

Although some committee Democrats were gentler in their questions, senators across the board expressed frustration with the administration’s response to events in eastern Ukraine, where armed, pro-Russian militants have seized dozens of government buildings and engaged in pitched battles with Ukrainian security forces.

Nuland repeated what she called the “four pillars” of administration policy that she outlined in a hearing last month — “supporting Ukraine, reassuring NATO allies” in eastern Europe, “creating costs for Russian behavior, and keeping the door open to de-escalation.”

So far, the administration has imposed asset freezes and visa bans on 45 Russian individuals close to President Vladi­mir Putin and 19 companies they control. It has temporarily sent air and naval assets, along with small numbers of troops, to fearful NATO allies close to Russia’s borders and said it would extend previously scheduled military exercises in the region. Europe has imposed its own targeted sanctions and agreed to participate in NATO reinforcements but has been slow to agree to the sectoral sanctions the administration has said are its next step.

It was uncertain whether Nuland’s “trigger” comments on the referendum referred to further sanctions against individuals or against economic sectors.

“Our resolve must be clear,” said Sen. Robert Menendez (D-N.J.), the committee chairman. In addition to steps already taken, he said, “NATO should begin preparations to station forces in Central and Eastern Europe . . . and should consider additional targeted sanctions” against Russia’s giant energy companies, Rosneft and Gazprom.

Several committee Republicans compared the administration’s response in Ukraine to its actions in Syria, where they charged it has waited too long to forcefully intervene.

“If we continue as we are, it’s going to be just like Syria,” said Tennessee Sen. Bob Corker, the senior Republican on the panel. “We let it get out of hand.”

“I don’t understand the thinking of waiting until the damage is done, and Russia has won, to put in place the things that matter,” he said.

Kerry made his own reference to war-torn Syria, where the Russian-backed government of President Bashar al-Assad has set elections for next month, in which Assad has announced he will run again.

“It’s very hard to reconcile that Russia is now making the argument that Ukraine ought to not have an election or postpone an election because of the violence that’s taking place,” Kerry said, “but Russia is whole hog behind having an election in Syria, where there is far worse violence. Reconcile that one for us, please.”

Lawmakers also questioned the administration’s reluctance to provide lethal aid to Ukraine’s military. Deputy Assistant Secretary of Defense Evelyn N. Farkas said that $18 million in nonlethal assistance — including uniforms, military meals and vehicles — had been provided. She said that additional requests are being considered but that “we will never be able to, in a short period of time, build up the Ukraine military to be a modern, agile force” capable of combating the Russians.

But, she said, the Defense Department could do more if it had more money.

Farkas declined to respond to persistent questioning by Sen. Barbara Boxer (D-Calif.), to “give me a number.”

After the open hearing, all 100 senators were invited to a closed-door briefing on Ukraine by Nuland, Farkas and representatives from Treasury, the Office of the Director of National Intelligence and the U.S. Agency for International Development.