Russia will take countermeasures to ensure its national security in the face of any NATO advancement toward Russian frontiers, a top Russian official warns.

“Any expansion of an organization like NATO in the direction of our borders will make it imperative for us to take countermeasures to ensure our own national security,” Kremlin spokesman Dmitry Peskov said on Tuesday.

“We must not forget that NATO is an organization that has been created during the time of confrontation and for confrontation, that’s why it cannot change its nature,” Peskov added.

He also said Russia regrets the plans declared by Ukrainian President Petro Poroshenko to continue its course on seeking accession to NATO.

The Ukrainian leader told a Tuesday meeting of the National Security and Defense Council in the capital, Kiev, that the country’s cooperation with the Western military alliance is strategic.

The council approved a new military doctrine on September 2 that urges Kiev to strive for NATO membership.

Also on Tuesday, Ukraine and NATO signed a set of agreements on strengthening military and technical cooperation. NATO Secretary General Jens Stoltenberg vowed to help the Kiev government defend itself against the pro-Russia forces in Ukraine’s troubled east.

Critics fear that NATO’s increasing presence in Ukraine could harm a truce agreement, dubbed Minsk II, which was reached between Ukraine’s warring sides at a summit attended by the leaders of Russia, Ukraine, France, and Germany in the Belarusian capital city of Minsk in February.

The agreement introduced measures such as a ceasefire – which officially went into effect on February 15 – the pullout of heavy weapons and constitutional reforms in Ukraine by the end of the year.

The shaky deal, however, failed to end the deadly violence in the mainly Russian-speaking regions of eastern Ukraine, with both sides trading accusations of breaching the ceasefire deal.

The conflict in Ukraine erupted in March 2014 following a referendum in the Black Sea peninsula of Crimea, in which people voted overwhelmingly for re-unification with Russia.

The situation, however, degenerated into a major armed conflict after Kiev dispatched troops to Luhansk and Donetsk in April 2014 in an attempt to suppress pro-Russia forces there.

Some 8,000 people have lost their lives and about 18,000 have been injured since April 2014, according to the UN.