Russia has built up troops along the border, raising fears of a major escalation in the long-running conflict in Ukraine's mainly Russian-speaking east.
Ukrainian President Volodymyr Zelenskiy has travelled to the country's eastern frontline, as German Chancellor Angela Merkel urged Vladimir Putin to reduce Russia's troop buildup near Ukraine.
Zelenskiy's visit came as fighting between the Ukrainian army and Russian-backed separatists intensified in recent weeks.
Russia has built up troops along the border, raising fears of a major escalation in the long-running conflict in Ukraine's mainly Russian-speaking east which has claimed more than 13,000 lives since 2014.
Images released by Zelenskiy's office showed the 43-year-old leader in the trenches clad in a helmet and bulletproof vest, handing out awards to Ukrainian soldiers and shaking their hands.
In a phone call with Putin on Thursday, Germany's Merkel urged the Russian president to reduce Moscow's "troop reinforcements" on the border "to de-escalate tensions".
Putin meanwhile "drew attention to the provocative actions of Kiev, which has recently been purposefully exacerbating the situation on the frontline," the Kremlin said.
Zelenskiy, who has urged NATO to speed up his country's membership into the alliance to support Ukraine, said he had visited positions where "the largest number of violations" of a ceasefire had been recorded, the presidency said.
Alliance members responded with calls for Kiev to continue military and defence reforms.
He thanked the soldiers "for protecting our land" and said "there is indeed an escalation" in eastern Ukraine.
"All commanders understand that snipers are targeting our guys," Zelensky said.
He said 26 Ukrainian troops had been killed since the start of the year, compared to 50 in all of 2020.
Ukraine last week accused Russia of massing thousands of military personnel on its northern and eastern borders as well as on the Crimean peninsula.
'Shot in the leg'
In Moscow, the Kremlin's pointman on relations with Ukraine and pro-Russia separatists, Dmitry Kozak, delivered a new warning to Kiev, saying Russia might "have" to step in to defend Russian speakers in the war-torn east, if need be.
But, he added, an escalation would be "the beginning of the end of Ukraine", describing that scenario for the ex-Soviet country as "not a shot in the leg, but in the face".
A new round of Ukraine talks is set for April 19, he added.
Support from Western allies
The United States is increasingly concerned about what it sees as escalating Russian aggression in eastern Ukraine, and is discussing Russian troop movements with its NATO allies, White House Press Secretary Jen Psaki said.
"Russia now has more troops on the border with Ukraine than at any time since 2014," Psaki told reporters, referring to the period when Russia annexed the Crimea region of Ukraine.
She said the United States was discussing its concerns about regional tensions and ceasefire violations with NATO allies.
In a statement on Thursday, Russian foreign ministry spokeswoman Maria Zakharova said NATO's support of Ukraine "does not contribute to security" and "the settlement of the conflict".
She added that Moscow was concerned about "financial and logistical support of the Ukrainian armed forces by NATO countries", as well as the alliance supplying lethal weapons and Western instructors training Ukrainian military personnel.
Analysts have been split over Russia's true intentions amid the latest escalation in tensions with Kiev, and some observers say Moscow may be testing US President Joe Biden's commitment to defend Ukraine.
In his first call with Zelenskiy last week, Biden affirmed Washington's "unwavering support" for Kiev in the conflict.
Zelenskiy, a television comedian and political outsider, came to power in 2019 on pledges to end the war.
Fighting in the conflict, which erupted after Russia's annexation of Crimea from Ukraine in 2014, subsided in 2020 as a new ceasefire agreement took hold last July.
But clashes, mainly involving artillery and mortar fire, have picked up again since the start of the year, with both sides blaming each other.
Ukrainian separatists are widely seen as having Russia's political and military backing, which Moscow denies.
Together with France and Germany, Ukraine and Russia are part of the Normandy format of countries that have sought to resolve the conflict since 2015 but have failed to end the fighting.
Kiev's Western allies have repeatedly warned Russia against taking further action and seeking explanations for its troop build-up on Ukraine's border.
The Kremlin has not denied the troop movements but insisted that Moscow was "not threatening anyone."