The US on Thursday said it was increasingly concerned over Russian militarisation along Ukraine's border, as Ukrainian president Volodymyr Zelensky travelled to the eastern frontline.

The number of Russian troops at the border with the former Soviet republic is now greater "than at any time since 2014," when war in eastern Ukraine first broke out and Russia seized the Crimea region, White House press secretary Jen Psaki said in Washington.

"The United States is increasingly concerned by recent escalating Russian aggressions in eastern Ukraine, including Russian troop movements on Ukraine's border," Psaki told reporters. "These are all deeply concerning signs."

A masked President Zelensky greets an army officer in the eastern town of Zolote
A masked President Zelensky greets an army officer in the eastern town of Zolote UKRAINIAN PRESIDENTIAL PRESS SERVICE / STR

Her comments came after the Ukrainian president visited the eastern frontline where fighting between the Ukrainian army and Russian-backed separatists has intensified in recent weeks.

Images released by Zelensky'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.

The long-running conflict in Ukraine's mainly Russian-speaking east has claimed more than 13,000 lives since 2014.

German Chancellor Angela Merkel on Thursday called on Moscow to reduce its troops buildup.

Map of Ukraine locating regions under separatist control and the Crimea, annexed by Russia.
Map of Ukraine locating regions under separatist control and the Crimea, annexed by Russia. AFP / Jochen GEBAUER

In a phone call with Russian President Vladimir Putin, Merkel asked him to scale down the 'troop reinforcements" on the border "to de-escalate tensions," her office said in a statement.

Putin meanwhile "drew attention to the provocative actions of Kiev, which has recently been purposefully exacerbating the situation on the frontline," the Kremlin said.

Zelensky handed out awards to Ukrainian soldiers in a frontline trench
Zelensky handed out awards to Ukrainian soldiers in a frontline trench UKRAINIAN PRESIDENTIAL PRESS SERVICE / STR

During his frontline visit, Zelensky 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, adding that 26 Ukrainian troops had been killed since the start of the year, compared to 50 in all of 2020.

In Moscow, the Kremlin's pointman on relations with Ukraine and pro-Russia separatists, Dmitry Kozak, on Thursday 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.

Jen Psaki: 'The United States is increasingly concerned by recent escalating Russian aggressions in eastern Ukraine'
Jen Psaki: 'The United States is increasingly concerned by recent escalating Russian aggressions in eastern Ukraine' GETTY IMAGES NORTH AMERICA / ALEX WONG

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.

Ukraine's Zelensky on frontline as Merkel urges Putin to pull back troops
Ukraine's Zelensky on frontline as Merkel urges Putin to pull back troops UKRAINIAN PRESIDENCY

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.

Ukraine last week accused Russia of massing thousands of military personnel on its northern and eastern borders as well as on the Crimean peninsula.

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.

The Kremlin has not denied the troop movements but insisted that Moscow is "not threatening anyone."

Zelensky this week urged NATO to speed up his country's request for membership in the alliance, saying this was the only way to end the conflict.

Alliance members responded with calls for Kiev to continue military and defence reforms.

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 committment to defend Ukraine.

In his first call with Zelensky last week, Biden affirmed Washington's "unwavering support" for Kiev in the conflict.

Zelensky, a television comedian and political outsider, came to power in 2019 on pledges to end the war.