WASHINGTON, Dec 21 (AFP) - Ukrainian President Volodymyr Zelenskyy on Wednesday (Dec 21) won vows of long-term US support including a new missile defence system during his first foreign trip since Russia's invasion, with President Joe Biden promising him, "You will never stand alone."

Three hundred days after Russian leader Vladimir Putin attacked Ukraine with hopes of swift conquest, Zelensky enjoyed a hero's welcome on a lightning trip to Washington, but he also made clear he would not accept any pressure to compromise.

Zelenskyy - whose media savvy and rugged demeanor have helped rally the world to Ukraine's cause - kept his trademark military fatigues rather than switching to a suit as Biden rolled out the red carpet, affectionately putting his hand over the wartime leader's shoulder as they entered the White House.

"You will never stand alone," Biden told him at a joint news conference.

"The American people are with you every step of the way and we will stay with you - we will stay with you - for as long as it takes."

Biden said that Americans "understand in our bones that Ukraine's fight is part of something much bigger".

"Together I have no doubt we'll keep the flame of liberty burning bright and the light will remain and prevail over the darkness."