— Russia and its sympathizers seized control of more Ukrainian military bases and facilities in Crimea on Monday while Moscow issued threatening statements about eastern Ukraine that signaled Russia’s intention to play a significant role in the country’s future.

At least four Ukrainian military bases, including one stocked with missiles, were overrun by armed men in uniforms who say they are members of local self-defense units, which are typically under the command of Russian military officers. The headquarters of the Ukrainian naval fleet had its electricity cut, and the director of a military hospital was ousted and a replacement installed by the pro-Russian militia that took over.

A foreboding sense of lawlessness is spreading ahead of a Sunday referendum in Crimea on whether to align with Russia or remain with Ukraine. Several activists critical of Russia’s presence in the Crimean region were reported missing. Residents of the regional capital, Simferopol, reported being visited by groups that stole or destroyed their passports, which are required as identification to vote.

Crimean officials are acting as if the referendum’s outcome is a foregone conclusion. On a Web site that the Crimean parliament started to drum up support for the referendum, an online poll showed that votes to join Russia outnumber those in favor of Ukraine almost three to one. Every article on the site put a positive spin on a union with Russia, including one that described the thousands of Russian tourists eager to vacation in Crimea.

Geoffrey R. Pyatt, the U.S. ambassador to Ukraine, said at a news conference in Kiev, the Ukrainian capital, that the United States will not recognize the results of the referendum.

“Any increase of the autonomy of Crimea should be done not under the barrel of a gun but in a clear, transparent and constitutional process,” Pyatt said.

Senior administration officials in Washington said the ball was in Russia’s court to de-escalate the situation, a prospect they acknowledged looked increasingly unlikely, or risk U.S. sanctions.

Officials said that Secretary of State John F. Kerry had declined an invitation to go to Russia and speak directly to President Vladimir Putin, and that he had instead sent a list of questions to Moscow over the weekend. Among the questions, delivered along with a proposal to discuss the answers with Russian Foreign Minister Sergei Lavrov, was whether Russia was prepared to take steps such as working to stop the referendum and open direct talks with the new government in Ukraine. Once Crimea was annexed, there would be little to talk about, said the officials, speaking on the condition of anonymity to discuss the bilateral communications.

The only response from Moscow appeared to come when Russian television showed Lavrov telling Putin that U.S. proposals for resolving the crisis were unacceptable because they recognized the interim Kiev government as legitimate.

Earlier in the day, the Russian Foreign Ministry issued a statement accusing masked men of firing at peaceful protesters in the eastern Ukrainian city of Kharkiv.

“Russia is outraged by the chaos, which is currently ruling in eastern regions of Ukraine as a result of the actions of militants of the so-called Right Sector amid utter connivance of the new authorities as they call themselves,” the statement said.

Witnesses have described very different scenarios, accusing pro-Russian demonstrators of attacking Ukrainian loyalists. Pro-Russian sentiment runs high in eastern Ukraine, as it does in Crimea.

On Monday, Vitali Klitschko, a politician and former heavyweight boxer who helped organize the protests against now-ousted leader Viktor Yanukovych and is running for president, visited Kharkiv, addressed a pro-Kiev crowd and was pelted with eggs by pro-Russian demonstrators.

Any ambiguity about who is in control of Crimea is lost on the ground. It is not the Ukrainian government.

Vladislav Seleznyov, a Ukrainian Defense Department spokesman in Crimea, said that 200 armed men in uniform arrived in 14 trucks around 1:30 a.m. Monday at an army missile base in Chernomorskoye, in western Crimea, and ordered the troops on the installation to give up their weapons. The Ukrainian soldiers refused, but they stored their weapons in an arsenal on the base and left, Seleznyov said.

Similar reports of takeovers came from other Ukrainian military installations, including a navy facility for radio communications near the Black Sea port of Sevastopol, a military hospital in Simferopol and two bases around the Tatar village of Bakhchysaray, Seleznyov said.

Several activists who oppose Crimea returning to Russia have been reported missing.

Igor Kiryuschenko, who heads the Crimean branch of the Ukrainian Republican Party, is thought to have been abducted one day after he took part in a pro-Ukrainian rally.

Colleagues reported that he was last heard from late Monday morning, when he called his office and told the secretary, “Goodbye. They have come for me.” He has not answered his cellphone since then.

Activists from AutoMaidan, a group of car owners who drove around Ukraine when the uprising against Yanukovych began, said a member of their group and another woman traveling with her had been detained at a checkpoint as they tried to enter Crimea overland Sunday afternoon.

Sergey Hadzhynov, one of the activists who made it to Crimea, said witnesses reported seeing the women kneeling on the ground, their hands bound behind their backs.

Amid the escalating tensions, NATO officials said Monday that the alliance would begin to fly reconnaissance missions over Poland and Romania to peer into Ukraine. Although such flights are fairly routine, the announcement appeared calculated to remind Russia that its actions are being watched.

The United States has called on Russia and Ukraine to show restraint, but Ukrainian Foreign Minister Andrii Deshchytsya suggested Monday that it is becoming more difficult.

“We have to admit that our life now is almost like . . . a war,” Deshchytsya said, speaking in English. “We have to cope with an aggression that we do not understand.”

Lally reported from Moscow. Pam Constable in Bakhchysaray, Anthony Faiola in Kiev, Anne Gearan and Karen DeYoung in Washington, and Griff Witte in London contributed to this report.