The army of the aggressor nation is ready at any moment to launch a massive invasion, and this is why Ukraine faces colossal challenges to strengthen its defensive capabilities, Ukrainian President Petro Poroshenko has said.

"Ukraine's State Border Service has many functions and tasks. The most important of them – is defending Ukraine's eastern borders from Russian invaders," Poroshenko said during ceremonies marking the 100th anniversary of the founding of Ukraine's State Border Service on Friday in Kyiv.

Poroshenko said during his speech that Ukraine's military is among the best among non-NATO countries and reiterated arguments for Ukraine to join the military bloc.

"We are knocking on their doors, and our hands are not empty," Poroshenko said, adding that it is important to continue feeling the ground under one's feet in order to avoid getting dizzy.

"For us Russia's armed forces represent the army of the aggressor country, which illegally annexed Crimea and occupied part of Donbas… and which is ready at any moment to launch a full-fledged invasion of Ukraine. That's why we face a massive challenge to strengthen the military preparedness of our country," the president said.