WASHINGTON (AFP) — Ukraine's President Volodymyr Zelensky will meet United States leader Joe Biden and address Congress in Washington on Wednesday, during a visit the White House said will send Russia a strong message of Western unity.

The secretly arranged trip will be Zelensky's first outside Ukraine since Russian forces invaded in February and comes on the 300th day of a war that has seen Russian forces halted by a stubborn Ukrainian army backed by Western arms, forced to retreat from captured territory and struggling to avoid further setbacks.

The visit will "underscore the US' steadfast commitment to supporting Ukraine for as long as it takes," White House spokesperson Karine Jean-Pierre said in a statement.

Zelensky flies to the US after a risky visit to the frontlines in Bakhmut, where both sides have endured heavy tolls in constant shooting and shelling over the past two months.

Brutal trench warfare and artillery battles around Bakhmut — once known for its vineyards and salt mines — have flattened large portions of the city and its surroundings.

Patriot missiles

Zelensky will visit the White House where Biden is to announce a new arms package worth almost $2 billion that a senior administration official said includes Patriot air defense missiles.

Patriot missiles are seen as crucial to help Kyiv fend off Russia's punishing missile and drone attacks on its infrastructure.

Zelensky is then expected to address a joint session of Congress, which House Speaker Nancy Pelosi said would be "a very special focus on democracy."

The visit was quietly planned, beginning with a phone call between Biden and Zelensky on 11 December, followed by a formal invitation one week ago and confirmation of the visit on Sunday.

Details of the visit leaked on Tuesday afternoon, with officials stressing to US media that there were still security concerns over Zelensky's travel.

In White House talks, Biden and Zelensky will discuss the arms and training provided by the US and allies, sanctions and other pressure on Russia, and economic and humanitarian aid that Ukraine needs, the senior White House official said.

"They will discuss every element of this conflict, including the situation on the battlefield and including the question of where the war goes from here," the official said.