Ukraine signed a landmark trade and economic pact with the European Union on June 27. Ukrainian President Petro Poroshenko said it is the "most historic day" for Ukraine since it gained independence. (European Union)

Shrugging off Russian threats and a burgeoning civil war, Ukraine signed a landmark trade deal Friday binding it to the European Union, a monumental step that came in defiance of months of Kremlin efforts to prevent the country from turning westward.

The move prompted a top Russian diplomat to warn immediately of “serious consequences” for Ukraine. A cease-fire that has brought some measure of calm to the country’s roiling east was extended until Monday, and E.U. leaders hinted that they would slap more sanctions on Russia if it does not take steps to achieve peace by that deadline.

The document signed Friday was the same one that was rejected in November by Ukraine’s then-president, Viktor Yanukovych. That decision sparked months of protests by pro-Western Ukrainians, a crackdown by Yanukovych and his eventual ouster in February, generating the highest tensions between the West and Russia since the Cold War.

More than 100 protesters died in Kiev under the blue and yellow banner of the European Union when they took to the streets to demand that Yanukovych reconsider his last-minute decision — made under heavy Russian pressure — to reject the agreement. Hundreds more Ukrainians and dozens of Russians have died in violence in eastern Ukraine since April, when pro-Russian separatists seized government buildings and territory in an effort to align themselves with Russia rather than the European Union.

More than 160,000 Ukrainians have been displaced from their homes since the start of the conflict, the U.N. refugee agency said Friday.

Ukraine's President Petro Poroshenko, center, poses with European Commission President Jose Manuel Barroso, left, and European Council President Herman van Rompuy at the E.U. Council in Brussels. (AFP/Getty Images)

Friday is “maybe the most important day for my country after independence day” following the 1991 breakup of the Soviet Union, Ukrainian President Petro Poroshenko said as he signed the deal in Brussels, using the pen he said Yanukovych would have used to sign it in November. “All of us would have wished to sign the agreement under different, more comfortable circumstances. On the other hand, the external aggression faced by Ukraine is another strong reason for this crucial step.”

Two other former Soviet republics, Georgia and Moldova, also signed the telephone-book-thick trade deals with the European Union on Friday, in the face of Russian threats of tough consequences if they did so. The agreements will require them to enact economic reforms, as well as to be more transparent in how they operate — measures that may reduce the corruption that has plagued all three societies since they gained independence.

Russia has said it views the expansion of E.U. ties to its border as Western encroachment on a region that has long been within the Kremlin’s sphere of influence, particularly Ukraine, which Russians see as the cradle of their civilization. Russia has sought to enlist those countries in the Eurasian Union, its competing vision of an alliance based on values dominated by Moscow and free of Western influence.

“The anti-constitutional coup in Kiev and attempts to artificially impose a choice between Europe and Russia on the Ukrainian people have pushed society toward a split and painful confrontation,” Russian President Vladimir Putin said in Moscow on Friday, calling for a swift return to peace.

Russian Deputy Foreign Minister Grigory Karasin said the deal would “no doubt . . . have serious consequences,” Russia’s Interfax news agency reported.

E.U. leaders — along with those of Ukraine, Georgia and Moldova — have said that the deal does not constitute a challenge to Russia.

The agreements will open the vast 28-nation E.U. market, with its 504 million residents, to tariff-free exports from the three countries in exchange for gradual work toward bringing regulations up to European standards. Leaders hope to follow the model of Poland and the Baltic nations, former Eastern bloc states that are now E.U. members and whose economies have grown significantly in the 23 years since the breakup of the Soviet Union. Ukraine, Georgia and Moldova, by contrast, have struggled.

The European Union has said that Ukraine could boost its annual trade by $1.4 billion through the deal, although that would be offset by the tariffs Russia has threatened to impose.

The agreement makes no promises of eventual full E.U. membership, a step the three countries have said they want to take. E.U. leaders have been cautious about committing to that measure, which would mean opening their labor market to the countries’ citizens. With 46 million residents, Ukraine is more populous than all but five of the E.U. countries.

Early Saturday, Poroshenko extended a temporary cease-fire until Monday night after a Friday deadline passed without a peace deal.

E.U. leaders decided against new sanctions against Russia, but they gave Moscow until Monday to push rebels toward peace, suggesting they may impose sanctions if it fails to do so. They called for Russia to help implement Poroshenko’s peace plan, ensure that separatists hand back captured Ukrainian border checkpoints and release a final team of captured observers from the Organization for Security and Cooperation in Europe.

White House press secretary Josh Earnest said Friday that the Obama administration supports the E.U. demands. He would not say what would happen if those actions weren’t taken by Monday but said the likelihood of sanctions would increase.

“We have signaled a clear willingness to act with partners and allies to further isolate Russia,” he said. “Additional unhelpful action will lead to additional economic costs.”

Russia annexed Ukraine’s autonomous Crimea region in March after pro-Russian separatists there staged an independence referendum, and Kiev has accused Moscow of aiding the separatists in eastern Ukraine.

The E.U. agreements are “milestones in the history of our relations and of Europe as a whole,” European Council President Herman Van Rompuy said at Friday’s ceremony. “In Kiev and elsewhere, people gave their lives for this closer link to the European Union. We will not forget them.”

Russia has leaned hard on its neighbors not to sign the deals, called “association agreements.” It banned imports of Moldovan wine last year, cut off the flow of natural gas to Ukraine last week and said it would raise tariffs on imports from all three countries in response to the deals.

Those steps are likely to impose major economic hardships on the countries, which remain closely tied to Russia as an export market. Armenia, another former Soviet republic that was due to sign the agreement, reversed course in September after intense Russian lobbying.

Moldova and Georgia, like Ukraine, face pro-Russian separatist movements on their soil, and officials in all three countries have expressed fears that Russia will stoke tensions even further following the E.U. deal. Russia went to war with Georgia in 2008 in the breakaway region of South Ossetia, and Russian soldiers are stationed as peacekeepers in the breakaway Moldovan region of Transnistria.

“This is a civilizational choice,” said Oleksiy Haran, a professor of comparative politics at the National University of Kiev-Mohyla Academy, referring to Friday’s agreement. “Especially now, when you have Russian aggression against its strategic partner.”

Karen DeYoung in Washington contributed to this report.