US Secretary of State Antony Blinken kicked off his visit to Ukraine on Thursday by reaffirming Washington's commitment to supporting Kiev's sovereignty and territorial integrity following a massive troop buildup by Russia.

In the first visit to Kiev by a senior US official under President Joe Biden, Blinken opened the day by meeting with Ukrainian Foreign Minister Dmytro Kuleba and underscoring the need for greater reforms, even as he showed support against Moscow.

"I'm here really for a very simple reason, which is to, on behalf of President Biden, reaffirm -- strongly -- our commitment to the partnership between our countries, our commitment to Ukraine's sovereignty, territorial integrity and independence," Blinken told Kuleba.

US Secretary of State Antony Blinken disembarks at Boryspil International airport outside Kiev, Ukraine, at the start of a one-day trip
US Secretary of State Antony Blinken disembarks at Boryspil International airport outside Kiev, Ukraine, at the start of a one-day trip POOL / Efrem Lukatsky

He added that Washington will "work with you and continue to strengthen your own democracy, building institutions, advancing your reforms against corruption."

Kuleba told Blinken that Kiev "deeply appreciates" the US aid Ukraine has received to support its battle in the east against pro-Russian separatists, who are widely seen as having the Kremlin's military and political backing.

The one-day visit comes after Russia last month amassed 100,000 troops on Ukraine's borders, the biggest mobilisation since Moscow seized the majority-Russian peninsula of Crimea in 2014 and war broke out in eastern Ukraine.

A Ukrainian serviceman walks in a trench on the frontline with Russia-backed separatists near the town of Krasnogorivka, Donetsk region, on April 23, 2021
A Ukrainian serviceman walks in a trench on the frontline with Russia-backed separatists near the town of Krasnogorivka, Donetsk region, on April 23, 2021 AFP / Anatolii STEPANOV

Clashes in the east between the government and separatists have been intensifying since January, a bloody new phase in Europe's only ongoing military conflict which has claimed more than 13,000 lives.

Russia quickly announced a pullback after the latest buildup, leading some experts to believe President Vladimir Putin was testing the will of Biden while seeking to intensify pressure on Ukraine.

Ukraine's President Volodymyr Zelensky visits army outposts in the Kherson region, on the administrative border with Russia-annexed Crimea, on April 27, 2021
Ukraine's President Volodymyr Zelensky visits army outposts in the Kherson region, on the administrative border with Russia-annexed Crimea, on April 27, 2021 UKRAINIAN PRESIDENTIAL PRESS SERVICE / Handout

Blinken arrived late Thursday from London where he joined other foreign ministers from the Group of Seven wealthy democracies in condemning Russia's "irresponsible and destabilising behaviour" in Ukraine and elsewhere.

He will also meet with President Volodymyr Zelensky, who has renewed calls to speed up Ukraine's entry into the NATO alliance in the face of fears about Russia.

Western European nations, mindful of Russia's response, have opposed Ukraine's accession and the idea has met a cool response in Washington.

US Secretary of State Antony Blinken says the US "reaffirms" its commitment to Ukraine's sovereignty, territorial integrity and independence during his meeting with Ukraine's Foreign Minister Dmytro Kuleba, in a visible show of support following a massive
US Secretary of State Antony Blinken says the US "reaffirms" its commitment to Ukraine's sovereignty, territorial integrity and independence during his meeting with Ukraine's Foreign Minister Dmytro Kuleba, in a visible show of support following a massive troop buildup by Russia. AFPTV / Andriy PERUN

The United States has, however, earmarked $408 million in security aid for Ukraine this fiscal year.

Blinken will also join Metropolitan Yepifaniy -- head of the Ukrainian Orthodox Church, which has split from Russian domination -- in laying flowers at a memorial to soldiers killed in the eastern Donbas region.

With Biden in the White House, Ukraine will likely enjoy a more sympathetic ear than with his predecessor Donald Trump who was notoriously fixated on conspiracy theories about the country.

Trump held up aid to Ukraine to press Zelensky to dig up dirt on Biden, leading to the former president's first impeachment.

The scandal returned to the headlines just before Blinken's trip as US investigators raided the home of Trump's former lawyer, New York's ex-mayor Rudy Giuliani, who had aggressively pressed unfounded allegations of impropriety in Ukraine by Biden's son Hunter.

Blinken is sure to seek to turn the page on Trump's scandals but the Biden administration has also pressed Ukraine on good governance -- long a major concern for Western partners.

Ahead of Blinken's trip, the State Department criticised Ukraine for removing the head of state energy company Naftogaz, saying the shake-up showed "disregard for fair and transparent corporate governance practices".

Andriy Kobolev had reduced Ukraine's dependence on Russian gas deliveries and introduced reforms that improved the company's public image.

The US must show Kiev that Kobolev's dismissal has consequences, the independent Ukrainian news website Yevropeiska Pravda said in an editorial this week.

It said his removal negated "one of the few successful reforms" carried out in Ukraine and would put to rest hopes in Kiev that Biden might visit the country later this year.

Blinken's visit is "a very good signal of support for Ukraine", Yeliseev said.