US President Joe Biden is in the Ukrainian capital Kyiv, Ednews.net reports citing Reuters.
Mr Zelenskyy says the pair discussed long-range weapons, while Mr Biden says Ukraine will get a new military aid package worth $500 million to be announced on Tuesday.
In a statement from the White House, the President says he will announce "another delivery of critical equipment, including artillery ammunition" and added that there will be a new Russia sanctions package later this week.
The unannounced trip comes after the White House said last week that there were no plans for the president to cross into Ukraine during his visit to Poland this week, to mark the anniversary of Russia's invasion.
Ukrainian MP Lesia Vasylenko confirmed Mr Biden's visit and wrote on Twitter: "Welcome Mr. President!".
Polish prime minister Mateusz Morawiecki said on Sunday that he and Mr Biden will discuss the possibility of increasing US troop presence in Poland.
It follows a series of recent trips by Mr Zelenskyy to a number of Western allies including the UK and France.
In December, Mr Zelenskyy visited President Biden at the White House on his first trip out of Ukraine since the war began.