Ukrainian President Petro Poroshenko is pressing top European Union officials to keep sanctions pressure up on Russia while the EU seeks reassurance that Ukraine is doing its utmost to battle corruption.

Poroshenko and the leaders of the EU's main institutions began a summit in Brussels Thursday aimed at underlining the strength of EU-Ukraine ties and addressing sectors like democracy, energy, trade and security.

The EU has committed almost 3.5 billion euros ($3.7 billion) to Ukraine over the past two years. It also launched a mission aimed at chaperoning the former Soviet Republic toward democracy.

But endemic corruption has undermined progress. Two high-profile reformist politicians resigned early this month in frustration.

Meanwhile Russia-Ukraine ties are deadlocked over Russia's annexation of Ukraine's Crimean Peninsula and support for pro-Russian insurgents in eastern Ukraine.