US Secretary of State Antony Blinken expressed Washington's concerns over security in eastern Ukraine during a phone call with Ukrainian Foreign Minister Dmytro Kuleba on Wednesday.
The call aimed to affirm Washington's "unwavering support for Ukraine's sovereignty and territorial integrity in the face of Russia's ongoing aggression," the State Department said in a statement.
A ceasefire between Ukraine and Russian-backed separatists has been brought into question following a slew of recent violations.
US military chief contacts Russian counterpart
Ukraine and Russia have been embroiled in a deadly conflict since Russian forces annexed Crimea in 2014. Moscow continues to support separatist rebels fighting in Ukraine's eastern Donbass region.
Blinken gave his condolences for the four Ukrainian soldiers who were killed in fighting last week.It was the highest death toll since the two sides agreed to a ceasefire last July.
The chairman of the US Joint Chiefs of Staff, General Mark Milley, also contacted his Ukrainian and Russian counterparts on Wednesday. He did not disclose what was discussed in the phone calls.
Ukraine claimed that the four died after coming under fire from Russian mortars. Russia has denied having any military presence in the Donbass region.
Ukraine warns of Russia's troops
Pentagon Press Secretary John Kirby said that the US government was aware of Russian troop movements on the Ukrainian border. He called it a violation of the ceasefire agreement.
The Organization for Security and Cooperation in Europe (OSCE) recorded hundreds of ceasefire violations in the past few days, including 493 on March 26 alone.
"Russia's destabilizing actions undermine the de-escalation in tensions that had been achieved through an OSCE-brokered agreement back in July of last year,'' Kirby said.
Ukrainian Commander-in-Chief Ruslan Khomchak warned of a buildup of Russian forces near Ukraine's border on Tuesday.
Kirby said that Washington was discussing the situation with NATO allies and would "reach out to Russia to request an explanation for these developments."
Moscow said on Wednesday that it was concerned by the increasing hostility in the east of the country and feared that Ukrainian forces could trigger another conflict with the pro-Russian separatists.
ab/dj (AP, Reuters)