(Corrects spelling in paragraph 4 of Ukrainian town to Irpin instead of Iprin)

(Reuters) -Republican presidential candidate Chris Christie visited Ukraine on Friday and met with President Volodymyr Zelenskiy as he underscored strong U.S. support for Kyiv's fight against Russia.

Christie, once an ally of former President Donald Trump, is now challenging him for their party's 2024 presidential nomination - and drawing a stark contrast on Ukraine with the front-runner.

Christie's visit to Ukraine came a day after Trump returned to Washington to plead not guilty to federal charges he orchestrated a plot to try to overturn his 2020 election loss.

The former New Jersey governor met with Zelenskiy at the presidential palace after visiting a mass grave in Bucha, a site where Ukraine accuses Russian troops of having committed atrocities, and touring damage in Irpin. Both towns were retaken by Ukrainian forces in 2022 as Russian invasion forces abandoned their attempt to seize Kyiv, the capital.

Christie also toured a child protection center in Kyiv.

His message during Friday's visit was clear: the U.S. supports and should continue to support Ukraine.

That stance sets Christie apart from some of the other candidates including Trump, who has been sharply critical of U.S. support of the war.

Florida Governor Ron DeSantis, a distant second in public opinion polls, this year suggested that the war was simply a "territorial dispute" before backtracking and labeling Russian President Vladimir Putin a war criminal.

Another candidate, entrepreneur Vivek Ramaswamy, has called for an immediate end to the war and for Russia to keep its territorial gains.

At a candidates forum in Iowa last month, former Vice President Mike Pence and U.S. Senator Tim Scott of South Carolina both argued that it remains vital for the United States to push back against Russian aggression.

The United States has provided billions of dollars worth of weapons to Ukraine following Russia's February 2022 invasion.

Trump, who left office in 2021 after one four-year term, was impeached in 2019 on allegations he pressured Ukraine to investigate his Democratic rival, Joe Biden. Senate Republicans acquitted him of those charges.

(Reporting by Doina Chiacu and Kat Jackson in Washington; Editing by Jonathan Oatis)