KYIV, Ukraine (AP) — Ukraine’s military said Tuesday that two of its servicemen were killed within 24 hours in the country's east, where Ukrainian forces have been fighting Russian-backed separatists since 2014 and where tensions have intensified in recent weeks.

The two military members sustained fatal gunshot wounds as a result of a “hostile fire," the Joint Forces Operation reported.

Firefights have occurred sporadically since tensions in eastern Ukraine escalated in late February. Ukraine's military reported on March 26 that four of its soldiers died in a mortar attack, the most significant violation this year of a ceasefire that had led to a decline in fighting.

In the past 24 hours, seven firefights took place along the line of contact, the Ukrainian military said.

More than 14,000 people have died in the eastern Ukraine conflict since 2014.

Officials in Kyiv and the West expressed concerns over reports of a troop buildup along Russia's borders with Ukraine. Ukrainian President Volodymyr Zelenskiy accused Russia last week of “muscle-flexing in the form of military exercises and possible provocations along the border."

Kremlin spokesman Dmitry Peskov said Monday that the troop movements “shouldn’t cause the slightest concern” and that Russia doesn’t pose a threat “to any country in the world, including Ukraine.”

Peskov said “the Russian army is moving on the Russian territory in directions it deems necessary...in order to ensure security of our country.”

But Zelenskiy, in a phone conversation with U.K. Prime Minister Boris Johnson late Monday, argued that “Russia’s recent actions pose a serious challenge to the security of Ukraine, NATO member states and the whole of Europe," according to a readout of the call released by Kyiv.

A readout from Johnson's office said the two leaders discussed the U.K.’s “significant concerns about the recent Russian military activity on Ukraine’s border and in illegally-annexed Crimea.”

NATO Secretary General Jens Stoltenberg said he called Zelenskiy on Tuesday to express the bloc's “serious concerns” about Russia's military activities.

“NATO firmly supports Ukraine’s sovereignty and territorial integrity. We remain committed to our close partnership,” Stoltenberg tweeted.

Zelenskiy, in turn, stressed the importance of Ukraine joining a program for prospective NATO members, saying that participating in the alliance's Membership Action Plan program could be “a real signal for Russia.”