SIMFEROPOL, Ukraine — Tensions mounted on the eve of a secession referendum in Crimea as helicopter-borne Russian forces made a provocative incursion just beyond the peninsula’s regional border to seize a natural gas terminal while US and European officials prepared sanctions to impose on Moscow as early as Monday.

The military operation by at least 80 troops landing on a slender sand bar just across Crimea’s northeast border seemed part of a broader effort to strengthen control over the peninsula before a vote Sunday on whether its majority Russian-speaking population wants to demand greater autonomy from Ukraine or break completely and join Russia. Whatever its goals, it sent a defiant message to the United States and Europe and underscored that a diplomatic resolution to Russia’s recent takeover of Crimea remains elusive.


The raid came as US and European diplomats essentially forced Russia to veto a UN Security Council resolution declaring the Sunday referendum illegal. Western diplomats hoped the result would reinforce Russia’s growing international isolation. Russia cast the only vote against the resolution; even China, its traditional ally on the council, did not vote with Moscow but abstained, an indication of its unease with Russia’s violation of another country’s sovereignty.

US and European officials readied lists of Russians to penalize after the referendum, including possibly vital members of President Vladimir Putin’s inner circle. Among the Russians under consideration for Western sanctions, according to officials, are Sergei K. Shoigu, the defense minister; Sergey B. Ivanov and Vladislav Surkov, two of Putin’s closest advisers; Alexei Miller, the chief executive of Gazprom, the state energy giant; and Igor Sechin, head of the oil company Rosneft.

The sanctions would ban the targets from traveling to Europe or the United States and freeze any assets they had in either place. Western officials said they do not plan to sanction Putin himself, at least at this point, because he is a head of state, nor do they intend to target Sergey V. Lavrov, the foreign minister, because he needs to travel if there are any future diplomatic talks.


Moreover, US and European officials said President Obama and his European counterparts may not start with the list of Putin confidants in whatever sanctions are imposed immediately after the referendum, so as to have the means to further escalate their response should Russia continue to press its seizure of Ukrainian territory. Instead, they may start with lower-level officials, military leaders, business tycoons, or parliamentarians.

Obama’s Cabinet secretaries and top advisers huddled in the White House on Saturday to discuss their strategy, joined by Secretary of State John Kerry.

The degree of sanctions and the exact timing may depend on how Moscow reacts immediately after the referendum, which is almost universally expected to approve seceding from Ukraine and becoming part of Russia, officials said. If Putin moves promptly to initiate annexation, that would trigger immediate action, but if he holds back and leaves room for talks, Washington and Brussels may defer.

Russia left little impression of backing down Saturday. Russian forces made a show of added strength here in Simferopol, the regional capital, stationing armed personnel carriers in at least two locations in the city center and parking two troop carriers outside the headquarters of the election commission.

The more provocative move, however, was the seizure of the gas terminal near a town called Strelkovoye, which drew new threats of a military response from the Ukrainian government. Until now, it has refrained from responding in force to Russian actions, but it sent troops Saturday to surround the gas terminal, though there were no immediate indications of any shots being fired, according to a Ukrainian news service quoting local police.


In Kiev, the Foreign Ministry said

Ukraine “reserves the right to use all necessary measures’’ to stop what it called “the military invasion by Russia.’’

The White House suggested the move only increased the likelihood of sanctions.