President Obama said Wednesday that the United States and its allies “will be forced to apply a cost” to Russia if it does not back down from “violations of international law and its encroachments on Ukraine.”

With Ukraine’s interim prime minister, Arseniy Yatsenyuk, by his side in the Oval Office, Obama said he and the international community reject what he called a “slapdash referendum” to be held Sunday in the Russian-occupied Ukrainian region of Crimea.

“We will never surrender,” Yatsenyuk said. “Ukraine is and will be a part of the Western world,” he added, noting that his government expects to sign an association agreement with the European Union by the end of next week.

Obama’s meeting with Yatsenyuk, a symbolic show of strong U.S. support for Ukraine, came as Secretary of State John F. Kerry prepared to meet with Russia’s top diplomat in London on Friday in a last-ditch diplomatic effort to head off the deepening confrontation with Moscow.

“There’s another path available” for Russia, “and we hope that President [Vladimir] Putin is willing to seize that path,” Obama said. “But if he does not, I’m very confident that the international community will stand strongly behind the Ukrainian government in preserving its unity and integrity.”

Even as they continued to offer Russia a diplomatic exit, officials made clear that they doubt Putin intends to take it. Instead, both the administration and leading Western governments were closely coordinating sanctions they plan to impose as early as Monday, immediately after the Crimea referendum.

“Almost a week ago, we said that if [the Western effort] wasn’t successful within a few days, we’d have to consider a second stage of sanctions,” German Chancellor Angela Merkel said Wednesday during a visit to Poland. The E.U. has already imposed visa bans on some former Ukrainian officials.

“Six days have gone by since then, and we have to recognize . . . that we haven’t made any progress,” Merkel said, according to the German news service Deutsche Welle. Both the E.U. and the United States plan to freeze assets of Ukrainian and Russian “individuals and entities” deemed responsible for political upheaval and corruption in Ukraine.

Meanwhile, the Defense Department announced Wednesday that it was sending 12 F-16 fighter jets and 300 personnel to augment an existing U.S. aviation detachment in Poland. Last week, an additional six U.S. F-15s and two refueling aircraft were sent to Lithuania. Two NATO AWACS planes are surveilling Ukraine and the Black Sea region to monitor possible Russian troop movements.

In addition to calling for the cancellation of the referendum on whether Crimea, with its majority ethnic-Russian population, wants to become part of Russia, the West wants Moscow to withdraw troops and to open talks with the Ukrainian government.

With no response from Russia — and passage of the referendum seen as assured — U.S. and European planning is now focused on what will happen after the vote Sunday. Any move by Putin to annex Crimea would probably be met with a new round of sanctions.

A statement by the Group of Seven industrialized countries criticized the “intimidating presence of Russian troops” in Crimea ahead of the hastily arranged secession vote and said that “Russian annexation . . . would be a clear violation of the United Nations Charter” and other international commitments.

Annexation, the group said, “could have grave implications for the legal order that protects the unity and sovereignty of all states. Should the Russian Federation take such a step, we will take further action, individually and collectively.”

In testimony before a House Appropriations subcommittee, Kerry said he was unwilling to “go into all of the sanctions. We’ve been pretty explicit about visa sanctions, banking sanctions, targeted business sanctions, individual kinds of sanctions.”

“I don’t want to go into all the detail except to say this,” Kerry said. “It can get ugly fast if the wrong choices are made, and it can get ugly in multiple directions.”

It was unclear whether Russia planned to “annex Crimea,” Kerry said. “They may well, but they may have the referendum, have the vote, and not move” in the Russian Duma, or parliament, “to do the other things.”

Short of annexation, Crimea — and the West — may be left in a sort of limbo.

Yatsenyuk said Ukraine plans no punitive moves against Crimea, such as cutting off water or electricity, no matter the outcome of the referendum. “Crimea is an integral part of Ukraine,” he said during a speech at the Atlantic Council after his meeting with Obama. “This is our territory, and they are our citizens.”

“If Russia moved further,” he said, apparently referring to fears that Russian troops might expand into eastern Ukraine, “this would definitely undermine the entire global security.” Only Putin knows how far he plans to take the confrontation in Ukraine, Yatsenyuk said. But, he alleged, Russia has contingency plans for occupying the entire country.

In addition to his White House visit, Yatsenyuk also had meetings on Capitol Hill, with the International Monetary Fund and, on Thursday, at the United Nations.

While they decide how to respond to Russia, the administration and its allies took additional steps this week to boost Ukraine’s foundering economy and solidify the government there as it moves toward a presidential election on May 25.

Obama called on Congress to quickly pass legislation, introduced Wednesday in the Senate, authorizing a $1 billion loan guarantee and sanctions against Russia in addition to those put forward by the president in an executive order last week. The House has passed its own version of the loan guarantees.

In a fact sheet detailing increased cooperation with Ukraine, the administration said it would reactivate a Strategic Partnership Commission, convene a U.S.-Ukraine business summit and hold bilateral defense consultations in Kiev within the next month.

The E.U. association agreement Yatsenyuk referred to in his remarks with Obama is the same document that his pro-Russian predecessor, Viktor Yanukovych, refused to sign. That decision was a major catalyst for massive, pro-Europe demonstrations. Yanukovych fled the country and is currently in Russia.

Russia has called the takeover by an interim government a coup and said it is protecting ethnic Russians and Russian-speakers in Crimea from “ultranationalist extremists.”

Ed O’Keefe contributed to this report.