US has ‘unwavering’ commitment to Ukraine: Biden

US President Joe Biden has raised concerns with his Russian counterpart about the military build-up in Ukraine where tensions are escalating.

Biden emphasized to President Vladimir Putin on Tuesday that Washington had "unwavering" commitment in its support for Ukraine.  

The US president "emphasized the United States’ unwavering commitment to Ukraine’s sovereignty and territorial integrity," according to a White House readout of the call.

However, he also asked Putin to reduce the tensions which Washington says are stirred by the Russian military build-up on Ukraine's border, proposing to hold a summit between the two leaders to address "the full range of issues" disputed between them.

In his second phone call to Putin since taking the helm as US president, Biden suggested to his counterpart that the two men meet in a third country to build "a stable and predictable relationship" between Washington and Moscow.

"The president voiced our concerns over the sudden Russian military build-up in occupied Crimea and on Ukraine’s borders, and called on Russia to de-escalate tensions," the White House said in a statement.

"President Biden also made clear that the United States will act firmly in defense of its national interests in response to Russia's actions, such as cyber intrusions and election interference," it added.

Kremlin: Biden asked Putin to help normalize relations

The Kremlin confirmed the phone call between the two leaders, adding that Biden had asked Putin to help normalize bilateral relations.

New START arms control, Iran's nuclear program, Afghanistan and climate change were other issues brought up in the phone call, according to the Kremlin.

Biden proposed a high level summit, it said without giving further information.

Earlier on Tuesday, US Secretary of State Antony Blinken, in Brussels for talks with NATO leaders and Ukraine's foreign minister, echoed Biden, saying Washington stood firmly behind Ukraine.

Blinken said he would discuss Kiev's bid to join the Western military alliance in his talks with NATO leaders.

NATO leaders have gathered to discuss the recent spike in violence in Ukraine’s eastern Donbas region. The Crimean peninsula's separation from Ukraine and attachment to Russia in 2014 referendum, met with Western opposition and US sanctions.  

In response, Moscow described the United States as the Russians' "adversary", using an old term that was used before in the Cold War era.

"The United States is our adversary and does everything it can to undermine Russia's position on the world stage," Deputy Foreign Minister Sergei Ryabkov was quoted as saying by Russian news agencies on Tuesday.

Biden on Friday had expressed strong US support for Ukraine in a call with the country's president, Volodymyr Zelenskiy.

"President Biden affirmed the United States’ unwavering support for Ukraine’s sovereignty and territorial integrity in the face of Russia’s ongoing aggression in the Donbass [sic] and Crimea," the White House said.

Biden emphasized his administration’s commitment to a strategic partnership with Ukraine and support for Zelenskiy’s anti-corruption plans and reform agenda.

"The leaders agreed these reforms are central to Ukraine’s Euro-Atlantic aspirations," the statement said. Zelenskiy said on Twitter he was glad to speak with Biden and appreciates U.S. support on different levels.

"We stand shoulder to shoulder when it comes to preservation of our democracies. My commitment to transform [Ukraine], improve transparency & achieve peace is strong. The American partnership is crucial for Ukrainians," Zelenskiy said.

Kiev and Moscow have traded blame in recent weeks for the spike in violence in the Donbass, where Ukrainian troops and pro-Russian forces have been fighting since 2014.  

The armed confrontations began when a wave of protests in Ukraine overthrew a democratically-elected pro-Russia government and replaced it with a pro-West administration. The majority in those areas refused to endorse the new administration.

That new government then began a crackdown on the mainly ethnic Russians in the east, who in turn took up arms and turned the two regions of Donetsk and Lugansk — collectively known as the Donbass — into self-proclaimed republics.

Kiev and its Western allies accuse Moscow of having a hand in the crisis. Moscow, however, denies the allegations.

In the meantime, Russia is sending more than 10 navy vessels, including landing boats and artillery warships, from the Caspian Sea to the Black Sea to take part in naval drills near the country's borders with Ukraine, Interfax reported.

Russia’s Defense Ministry said last week that the fleet was moving to the area as part of the drills that involve approximately 15,000 personnel and will inspect combat readiness of the country’s naval forces by the end of this month.

“As part of the winter training check, more than 10 amphibious and artillery boats and vessels of the southern military district are conducting an inter-fleet move from the Caspian Sea to the Black Sea,” the ministry said.