British Prime Minister Boris Johnson says he has “significant concerns” about Russia’s military buildup near its border with Ukraine.

In a call with Ukrainian President Volodymyr Zelenskiy on April 5, Johnson also voiced “unwavering support” for Ukraine’s sovereignty and territorial integrity, Johnson’s office said.

He is the latest Western leader to speak with Zelenskiy in recent days amid reports of significant Russian troop movement near Ukraine’s border.

U.S. President Joe Biden spoke with Zelenskiy on April 2 about the buildup in his first call with the Ukrainian leader since taking office in January.

Fighting between Ukrainian government forces and Russia-backed separatists in two eastern provinces has spiked in recent weeks despite a cease-fire agreement reached in the summer.

In the meantime, Russia has been moving troops toward its border with Ukraine in what Moscow says is an exercise. The United States has called it an attempt to intimidate Ukraine.

During the call with Johnson, Zelenskiy asked the United Kingdom and its allies to beef up their presence in its neighborhood, according to a readout of the conversation published by the Ukrainian president’s office.

“Russia’s recent actions pose a serious challenge to the security of Ukraine, NATO member states, and the whole of Europe,” Zelenskiy said.

He also urged Western nations to impose tougher sanctions on Russia for its destabilizing activities and invite Ukraine into NATO’s Membership Action Plan. The action plan is a NATO program of assistance designed to help countries wishing to join the alliance meet its criteria.

Russia is opposed to Ukraine joining the military alliance and recently warned NATO countries against sending troops to support Ukraine.

Ukraine has been battling the Russia-backed separatists in a low-simmering war since 2014, when protesters in Kyiv toppled Kremlin-friendly President Viktor Yanukovych.

More than 13,000 people have been killed in the ensuing seven years. Russia is demanding Ukraine give the separatist-controlled regions greater autonomy, which would effectively prevent the country from joining NATO.

Ukraine has blamed the Russia-backed separatists for the recent spike in hostilities, while Moscow has pointed the finger at Kyiv.

Russia, which forcibly annexed Crimea in 2014 after long denying the presence of its troops there, has consistently denied involvement in the fighting in eastern Ukraine’s Luhansk and Donetsk regions despite significant evidence to the contrary.

Some analysts have suggested that the recent actions may be Russia’s way of testing the new Biden administration’s commitment to Ukraine.

With reporting by AFP and Reuters