(CNN) -

Ukrainian President Petro Poroshenko said early Tuesday that his country will not renew a cease-fire with pro-Russian separatists, vowing instead "we will advance, and we will liberate our land."

The fragile cease-fire expired at midnight Monday -- hours after Poroshenko spoke by phone with Russian President Vladimir Putin, German Chancellor Angela Merkel and French President Francois Hollande.

The cease-fire -- agreed upon earlier this month amid a volatile political crisis -- raised hopes that Ukraine could be moving back from the brink of full-fledged civil war.

The crisis has its roots in former President Viktor Yanukovych's decision to shun a European Union Association Agreement last year and work with Russia instead. The move unleashed deadly strife that led to Yanukovych's ouster, Ukraine's loss of Crimea and a pro-Russia separatist rebellion.

Russia and Ukraine have been engaged in a tense standoff since the Russian annexation of Crimea in March, when Russia also massed troops along its western border with Ukraine.

After Monday's phone call, Poroshenko said his goal was peace, but said it takes the participation of all parties to maintain stability, noting violations of the cease-fire by pro-Russian separatists.

A statement from Putin's press office about the call said the Russian President "stressed the need to extend the cease-fire and also establish a reliable mechanism for monitoring" it.