A cease-fire in Ukraine is being put to the test as skirmishes break out between Ukrainian military forces and pro-Russia separatist groups.

The violence comes after Ukrainian President Petro Poroshenko signed a European Union free-trade deal that was the catalyst for the Euromaidan protests and after he extended the weeklong cease-fire to Monday at 10 p.m. Carloads of Ukrainians were reported fleeing eastern Ukraine to Russia this week, prior to Poroshenko’s extension.

Bloomberg reports, citing information from the Defense Ministry of Ukraine, three were killed and four wounded in a separatist mortar attack on a Ukrainian military position in Slovyansk, a city in the contested Donetsk Oblast. Aleksander Boroday, the leader of the Donetsk People’s Republic (DPR), one of the largest separatist groups, denies violating the cease-fire. Two other leaders in the region reportedly disregarded the truce.

The Ukrainian government says it reserves the right to end the truce before Monday if violence continues. The European Council, the political wing of the EU, called upon both parties to respect the cease-fire and urged Russia to “actively use its influence over the illegally armed groups … in order to achieve rapid and tangible results in de-escalation.”

The European Council calls on separatists to release all hostages, including observers from the Organization for Security and Co-operation in Europe (OSCE), return border checkpoints to the Ukrainian military, and continue with a peace plan with the Ukrainian government. Separatists released four OSCE observers on Friday after holding them for a month.

Georgia and Moldova joined Ukraine in the EU free trade deal. The deal allows for the free flow of goods between those countries and EU countries without tariffs as long as their goods meet EU trade standards. One pro-EU Ukrainian, Oleg Mityukhin, 48, came out to celebrate the deal in Kiev’s Independence Square, the heart of the pro-EU Euromaidan protests.

“I think there will be less corruption, there will be better quality goods, and it will be a push forward for the development of Ukraine,” he said.

President Poroshenko called it a “historic day for Ukraine,” and “the most important day for Ukrainian history after Independence Day.” He noted the violence and loss of life that went in to signing the trade deal, saying Ukraine “paid the highest possible price” to integrate with the EU.

Leaders of the DPR on Saturday said they would take immediate action to withdraw from the Ukrainian legislative bodies to distance the east from the economic changes resulting from the EU deal. Leaders say the east’s industrial economy is threatened by European integration. Andrei Purgin, the deputy prime minister of the DPR, told Itar-Tass the self-declared state would attempt to become a part of the Eurasian Economic Union (EEU), a customs union between Russia, Belarus and Kazahkstan.