Ukraine's President Petro Poroshenko told CNBC on Friday that his country will look to engage with Russia on a military level if it fails to de-escalate tensions in east Ukraine by Monday.
A ceasefire in east Ukraine is currently in place after months of violence in the region. It was due to end late on Friday, but Ukraine is expected to extend it until Monday.
"I think that the ceasefire will expire and Ukraine will defend ourselves with all methods we have in our disposal, including military," Poroshenko said at an EU summit in Brussels.
"Because we cannot accept that (they're killing) Ukrainian soldiers. That our check points are captured and the tanks, artillery system are going to Ukraine," he said.
Leaders from the European Union said they are ready to impose further sanctions on Russia if the Monday deadline is not respected. In a statement, the European Council also demanded the release of prisoners in east Ukraine and for officials to be granted control of three border checkpoints.
But despite the threats of military action, Poroshenko stressed he was a "president of peace."
"I use every chance to keep the peace in Ukraine. But if this peace initiative would be rejected, by Russia or by separatists… we should defend our country. This is very simple," he told CNBC.
His comments come on the day Ukraine signed a landmark trade and economic pact with the European Union. It marks a definitive move by Ukraine towards more integration with Europe – and away from Russia.
The refusal by Ukraine's former pro-Russian president, Viktor Yanukovych, to sign the pact sparked widespread protests that resulted in him fleeing the country, and eventually led to ongoing tensions with Russia.
Russia responded to news of the pact by saying that "grave consequences" would follow, according to Interfax news agency.
"Ukraine's signature... is a defeat for Russia, which had attempted to block these efforts, as it sought to build its own political/economic block," Timothy Ash, head of emerging markets at Standard Bank, said in a note.
He added that Russia's push for the Commonwealth of Independent States or Eurasian Union would now have "much less international standing in the absence of Ukrainian participation."
Georgia and Moldova also signed the deal, known as the Association Agreement and Deep and Comprehensive Free Trade Agreement (AA/DCFTA). Ash said the benefits for the countries signed up to the agreement included greater trade access, better investment and improvements to regulation.
But Ash warned there would likely be repercussions. "Russia is expected to retaliate by raising tariff and non-tariff barriers to Ukrainian trade, and announcing a program of import substitution away from Ukraine."
"This will be very disruptive to the Ukrainian economy, given that around one-third of trade had been orientated towards Russia – with a particular vulnerability for business in south-east Ukraine, currently the focus of separatist movements."