BRUSSELS, Belgium (CNN) -

Ukrainian President Petro Poroshenko on Friday signed a broad agreement of cooperation with the European Union, the same deal whose reversal set off a crisis in the nation.

The signing came hours before the President's office announced a three-day extension of a cease-fire. This is the latest step suggesting Ukraine may be moving back from the brink of full-fledged civil war -- though the situation remains volatile, with continued violence and the constant fear of yet more, cease-fire or not.

This tension has recent roots in then-President Viktor Yanukovych's decision to shun the EU Association Agreement last year and work with Russia instead. That move unleashed deadly strife that led to Yanukovych's ouster, the loss of Crimea and a pro-Russia separatist rebellion.

Sealing the deal may be the second-most important moment in Ukraine's history, Poroshenko said, after its independence from Russia.

He said the signing "shows how dramatically things can change in a short time, if the will of the people is strong enough."

And he paid tribute, on what he described as a "great day" in the nation's history, to Ukrainians who lost their lives when protests over closer ties with Europe turned bloody.

"The document we will sign today is not just political and economic, it is a symbol of faith and unbreakable will," he said.

"It is a tribute to people who gave their lives and health to make this moment happen, and it is the strongest reminder that today's Europe is and must be about people's determination to live in a better and safer world."

Poroshenko said that in signing the Association Agreement, he was making a unilateral statement that his country has underlined its choice of future membership in the EU.

And he signaled to Russia that its efforts to undermine the closer unification of Europe would fail in the face of Ukraine's determination to pursue its European dreams.

He used the same pen intended for use in November before Yanukovych turned his back on the agreement in favor of Moscow.

Russian reaction

Kremlin spokesman Dmitry Peskov said on Russian state TV that signing the EU deal is a "sovereign right" but that "Russia will undertake its measure if its market is negatively affected" by the agreement.

Such action probably would be protective trade measures intended to shield Russian producers and industry from an influx of potentially cheaper, better-quality goods from Europe.

Speaking at a Brussels news conference after the signing, Poroshenko said what was needed more than sanctions was a "real dialogue" with Russia. The EU and the United States have repeatedly warned Moscow that more economic sanctions could be imposed if it doesn't act to defuse the crisis.

"I really hope that finally now this dialogue will take place and we will have a real cease-fire and the implementation of my peace plan," Poroshenko said.

Speaking in Moscow, Putin also emphasized the need for a lasting truce, according to Russia's state-run news agency ITAR-Tass.

"Ukraine should embark on the path of peace, dialogue and accord. The priority is to conduct substantial talks between the authorities in Kiev and the southeast," Putin is quoted as saying.

"We stand for the complete termination of bloodshed on the whole territory of Ukraine, including along our borders," he said, adding that Russia is doing its best to contribute to the peace process.

Foreign Minister Sergey Lavrov on Friday called for Ukraine to carry out promised constitutional reforms and said Russia should be involved in consultations to safeguard the interests of Russian speakers in southeast Ukraine.