Ukraine accused Russia on Wednesday of massing troops near the border between the two countries and demanded that Moscow stop its “unacceptable military intervention” ahead of what Kiev calls an illegal referendum in Crimea on joining the Russian Federation.

Russia denied that it its massing forces or intends to invade the former Soviet republic on its southwestern border.

Andriy Parubiy, secretary of Ukraine’s National Security and Defense Council, told reporters in Kiev that Russia has deployed more than 80,000 troops, up to 270 tanks and 140 combat planes close to the border, creating the “threat of a full-scale invasion from various directions,” the Associated Press reported.

Parubiy said some Russian troops are as close as a two- or three-hour drive from the Ukrainian capital.

In Moscow, Deputy Defense Minister Anatoly Antonov denied that any military buildup was underway on Russia’s nearly 1,250-mile border with Ukraine.

The purported buildup came as interim Ukrainian Prime Minister Arseniy Yatsenyuk visited Washington on Wednesday to meet President Obama and enlist U.S. support in Ukraine’s standoff with Russia.

Obama warned Russia that the United States and its allies “will be forced to apply a cost” if Moscow does not end its incursion into Ukraine’s Crimea region and annexes the territory.

Calling Russia’s incursion into Crimea a violation of international law and the territorial integrity of Ukraine, Obama said the United States “will stand with Ukraine in ensuring that that territorial integrity and sovereignty is maintained.”

With Yatsenyuk seated alongside him in the Oval Office, Obama said: “We will continue to say to the Russian government that if it continues on the path that it is on, then not only the United States but the international community . . . will be forced to apply a cost to Russia’s violations of international law and its encroachments on Ukraine. There’s another path available, and we hope that President [Vladimir] Putin is willing to seize that path.”

Yatsenyuk told reporters that Russia has committed “unacceptable military intervention into a sovereign and independent state.” He expressed appreciation for U.S. support, adding: “We do believe that in the near future, the new Ukrainian government will be ready to deliver real changes, but in order to deliver these changes, we need to stop the Russian military.”

Earlier, the United States and its allies in the Group of Seven industrialized countries demanded that Russia stop efforts to split the Crimea region from Ukraine and warned of “further action, individually and collectively,” if a secession vote takes place on Sunday as scheduled.

In a statement, the group criticized the “intimidating presence of Russian troops” ahead of the hastily arranged secession vote. The statement came just hours before Obama hosted Yatsenyuk at the White House.

In Warsaw, German Chancellor Angela Merkel warned separately that a round of European Union sanctions against Russia were “unavoidable” if Putin failed to defuse the crisis over Ukraine, Bloomberg News reported. Merkel, visiting the Polish capital to meet with Prime Minister Donald Tusk, said Moscow has until March 17 to accept her proposal for a diplomatic “contact group” to resolve the crisis.

On his first visit to Washington as acting prime minister, Yatsenyuk met not only with Obama but with Secretary of State John R. Kerry, officials from the International Monetary Fund and others Wednesday, receiving a broad show of international support for his fledgling government.

Kerry said separately Wednesday that he would meet Russia’s foreign minister in London on Friday in what the administration described as a last-ditch diplomatic effort to head off a confrontation with Moscow over Ukraine.

Several U.S. senators plan to visit Ukraine this weekend to meet with the country’s leaders and show support for the new government in its standoff with Russia. Sen. John McCain (R-Ariz.) confirmed that he and “a large number” of senators from both parties would be making the trip. Aides later said seven senators would make up the delegation, including Senate Majority Whip Richard J. Durbin (D-Ill.), the second-ranking Democratic leader in the chamber.

In the G-7 statement, the United States, Japan, Germany, France, Britain, Italy and Canada said the Russian military takeover of Crimea made any referendum “deeply flawed.” They said the result would not be acknowledged.

Russia pushed into the Crimea region after Ukraine’s pro-Russian government was toppled last month following mass protests, and Russian forces have since consolidated their control of the peninsula, an autonomous part of Ukraine inhabited by an ethnic Russian majority.

The Crimean regional parliament has already voted to align with Russia, and officials there are treating the outcome of the vote as a foregone conclusion. The main airport has been closed to flights from the rest of Ukraine to prevent what Crimean authorities called “provocateurs” from entering ahead of the referendum.

“The annexation of Crimea could have grave implications for the legal order that protects the unity and sovereignty of all states,” the G-7 statement said. “Should the Russian Federation take such a step, we will take further action, individually and collectively.”

The United States and European nations are already considering a range of economic and other sanctions against Russia and are arranging financing to help prop up the new government in Kiev.

In Crimea, however, the rush to break away from Ukraine showed no signs of abating.

A Crimean official announced in a television broadcast that the regional government would soon take possession of Ukraine’s state-owned companies in the area, including an energy firm and railway, Reuters news service reported.

The Russian Interfax news agency reported that the civilian airport at Simferopol will remain closed until Monday — the day after the secession vote.

“Bearing in mind the possible influx of provocateurs, we have limited plane arrivals,” Crimean First Deputy Prime Minister Rustam Temirgaliyev told Interfax. “All limitations will be lifted after March 17 and the airport will resume its normal operation.”

Currently, the only flights operating are to and from Moscow. A flight from Kiev was turned back on Monday, and Turkish Airlines has suspended flights from Istanbul.

Russian troops are also continuing to expand their field infrastructure around Crimea, a region with deep historic and cultural ties to Russia.

According to Vladislav Seleznyov, a spokesman for the Ukrainian military, Russia has established a medical unit at an airbase near the city of Dzhankoy in northern Crimea, and more equipment is being moved in and installed on Wednesday. He said about 300 Russian troops are believed to be on the base. He said five Russian helicopters landed there Tuesday — one MI-8 and four MI-24 helicopters.

Russia on Wednesday also intensified the pressure on Ukraine’s new leadership, issuing an arrest warrant for the leader of the militant faction that toppled the government there and sparked the Russian move into Crimea.

The court order in Moscow was issued in absentia for Dmytro Yarosh, head of the Right Sector movement and deputy director of Ukraine’s Security Council. He was accused of inciting “terrorist operations” in parts of Russia. In a separate action, a Russian agency warned a local Web site for publishing statements by another Right Sector leader — a sign of Moscow’s determined push to cast the backers of the new government as extremists.

The right-wing movement provided much of the muscle when demonstrators in Kiev confronted police in the conflict that toppled Ukrainian President Viktor Yanukovych — who is now in Russia but has declared he will return and reassert control.

Russia has accused Yarosh of violating a Russian law prohibiting extremism in the mass media by publishing an appeal to militants in the North Caucasus of Russia “to step up extremist and terrorist operations against Russia.”

Yarosh has said the statement was placed on his Web site by hackers, suggesting Russia was behind the attack on his site.

Ukrainian officials have said they have no intention of detaining or extraditing Yarosh.

The Russian agency that supervises the media also issued a warning Wednesday to the news site for publishing an interview with Andriy Tarasenko, the head of Right Sector’s Kiev office. The article had a link to statements by Yarosh, which the agency said was a violation of the law against extremism.

Later in the day the Lenta editor resigned and was replaced by someone considered more acceptable to the Kremlin.

As Crimea grew more militarized and isolated Tuesday, hopes for a diplomatic solution to the Ukraine crisis looked increasingly faint. While NATO planes monitor Russian activity from Polish and Romanian airspace, European politicians said they were preparing to punish Russia with sanctions within days.

British Prime Minister David Cameron, speaking in Tel Aviv, said the first steps could include a travel ban targeted against “prominent” members of the Russian parliament.

European officials met in London on Tuesday to draw up penalties they could impose against Russia, likely to include asset freezes and visa restrictions, unless the country accepts a U.S. proposal to stop its expansion in Crimea and start discussions with Ukraine’s new government. Until now, Western efforts to curb Russia’s actions have focused on rhetoric and largely symbolic gestures, rather than measures that would cause meaningful pain in Moscow.

The European restrictions could mark a substantial escalation in a conflict that has pitted Russia against the West in a way not seen since the Cold War. But the Obama administration has refused to set a deadline for U.S. sanctions or indicate a specific Russian action that would trigger them. And analysts say that even tough sanctions are unlikely to force Russian President Putin to change course in Ukraine, given the depth of Russian interests there.

Meanwhile, Russian soldiers and the paramilitary “self-defense” units under the command of the Russian military continued their step-by-step takeover of Crimea. On Tuesday, they were in control and standing guard at several military bases, with Ukrainian troops cornered inside.

In the regional capital, Simferopol, witnesses reported that pro-Russian forces had taken over the prosecutor’s office and the railway administration. At the civilian airport, all Istanbul and Kiev flights were canceled, leaving only flights to and from Moscow in operation.

Branigin reported from Washington. Carol Morello in Sevastopol, Pamela Constable in Simferopol, Will Englund in Moscow, Griff Witte in London, and Howard Schneider, Ed O’Keefe, Anne Gearan and Karen DeYoung in Washington contributed to this report.