President Joe Biden called Ukrainian President Volodymyr Zelensky on Friday to pledge the United States’ “unwavering support for Ukraine’s sovereignty and territorial integrity in the face of Russia’s ongoing aggression in the Donbass and Crimea,” the White House announced.

Washington has cited Russian “aggression” amid the rising tensions in Ukraine as Russian troops and armor are rushing to the border region. Although Ukraine is not a member of NATO, President Biden has pledged American support of what he called “a strategic partnership” with Zelensky’s government “based on our shared democratic values that deliver justice, security, and prosperity to the people of Ukraine.”

Meanwhile, on Saturday, Ukraine’s military released a statement saying that its forces and NATO troops will hold joint military drills later this year. The planned military exercises, named Exercise Cossack Mace, will be conducted with over 1,000 troops from five different NATO countries.

In a thinly designed reference to the recent Russian troop build-up on Ukraine’s border, the exercise will include, “defensive actions […] followed by an offensive in order to restore the state border and territorial integrity of a state that has been subjected to aggression by one of the hostile neighboring countries.”

NATO is calling the exercise with Ukraine “routine.” Its exact dates are not yet specified but it is expected to take place this summer. “Exercise Cossack Mace is a routine exercise which is still in the planning stage,” a U.K. military spokesperson said in a statement. “It is due to take place in Summer 2021.”

Ukraine has been pushing for direct talks with President Biden for several weeks amid the increasingly bellicose actions by Russia along the Ukrainian border. Russian-backed separatists have been fighting the Ukrainian government since 2014. The recent escalation by Russia resulted in the breaking of a ceasefire agreement that was signed in July of last year, after having been put together by the Organization for Security and Cooperation in Europe (OSCE).

Four Ukrainian soldiers died and two others were wounded in the village of Shumi on March 26. Ukrainian military officials blamed the deaths of the four troops on a Russian mortar attack. Russian officials deny that they have any troops in the region. However, in a contradictory statement, Russian President Vladimir Putin acknowledged in 2015 the presence of Russian forces there “who carried out certain tasks including in the military sphere.” Putin has insisted that the only Russians in the region are “volunteers” who help the rebels but have no ties to the Russian military or the government.

Russian Forces Amass on Ukraine's Border

Read Next: Russian Forces Amass on Ukraine's Border

There are reportedly 3,000 Russian military advisers located within the Donbass region in support of the separatists who had been violating the ceasefire daily. Now, the Russians have allegedly moved 28 battalions, totaling 20,000-25,000 troops, on Ukraine’s eastern border.

To justify the increased presence of its troop on the border with Ukraine, Russia is blaming the escalation of tensions on NATO and the United States.

Kremlin spokesman Dmitry Peskov said that “the Russian Federation moves its armed forces within its territory at its own discretion.” He also added that Russia feels threatened by what it considers “increased activity of the armed forces of NATO countries” and others that “obliges us to be on the alert.”

Peskov then accused Ukrainian officials of staging “provocations” along the border.