REUTERS - It has become increasingly difficult for Ukraine to secure Western support in its fight against "Russian aggression" and a full-scale invasion from Russia cannot be ruled out, Ukraine's President Petro Poroshenko said on Tuesday.

Poroshenko said Europe's internal strength was being tested by external issues such as the migration crisis and militant attacks, and warned elections in 2017 could see political forces more willing to compromise with the Kremlin take power.

His comments underline Ukrainian concerns about losing support in its standoff with Russia over the annexation of Crimea in 2014 and an ongoing conflict between Ukrainian troops and pro-Russian separatists in the Donbass region.

The European Union is divided over whether to continue putting sanctions on Russia that have taken an economic toll on both sides. Ukrainian lawmakers have previously also voiced fears that the rise of populist sentiment in Europe and Great Britain's exit from the EU bloc could weaken support for Kiev.

"Ukraine will continue to need strong international support in the fight against Russian aggression," Poroshenko said at the opening of the September parliament session.

"But securing this support is becoming increasingly difficult for our diplomats due to different objective and subjective factors."

The conflict in Donbass has killed more than 9,500 people since separatist violence erupted two years ago, and a ceasefire agreement is routinely violated.

Tensions between Ukraine and Russia spiked in August after Russia accused Ukraine of planning attacks in Crimea.

As a result, while Poroshenko said that Ukraine is now safer than it was a year ago, he also warned that a full-scale invasion from Russia could not be ruled out.

"Over the course of the next year, political forces could come to power as a result of elections in several European countries that may not be extremist, but are inclined to compromise with the Kremlin," Poroshenko said.

"Europe still speaks with one voice, but there are also countries where the Russian accent is already too audible."