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Ukraine's President Volodymyr Zelensky was expected to visit Washington on Wednesday for a possible meeting with Joe Biden and an address to Congress - his first trip abroad since Russia invaded in February.

That unannounced trip came as Russian President Vladimir Putin planned to meet senior military officials Wednesday to weigh the results so far of Russia's war and set goals for next year after a series of battlefield defeats.

Zelensky's visit provisionally includes a meeting with Biden in the White House for an announcement of a fresh package of military aid for Ukraine, including advanced Patriot batteries to help defend against Russia's brutal missile attacks, US media reported.

The visit remained tentative and subject to security concerns.

House Speaker Nancy Pelosi told members of Congress in a letter that they should be in attendance on Wednesday evening "for a very special focus on democracy".

She would not confirm the visit to reporters late Tuesday, but said a visit by a "total hero" like Zelensky would "bring honor to the Congress of the United States".

In battling Putin, the Ukraine people "are fighting for democracy for all of us," she said.

Zelensky, who has led without rest for 10 months of brutal war with invading Russian forces, has spoken frequently to leaders and supporters around the world, but only by phone and video conference.

His planned trip acknowledges that the United States has been the most important supporter of the war effort, already providing an estimated $50 billion in total aid, including some $20 billion in security assistance.

From the frontline to DC

The trip comes after Putin made a visit to Belarus on Monday, stoking Ukraine worries that Russian forces could open a new attack from the north.

Belarus was the launch site of the original February assault on Kyiv, which was rebuffed by Ukraine forces in an extraordinary defeat for Moscow's forces in the opening weeks of the war.

On Tuesday, Zelensky, 44, visited Ukrainian troops in Bakhmut, now the epicenter of fighting, where he described the war-battered city as a frontline "fortress".

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.

"Here in Donbas, you're protecting all of Ukraine. They will do everything they've done here in other towns of our country, because they don't want anything Ukrainian to exist, I'm sure of that," Zelensky told his troops.

"This is not just Bakhmut, this is fortress Bakhmut," he said, handing out honours to Ukraine servicemen.

Soldiers gave Zelensky a Ukrainian flag with their names signed on it and asked him to give it to Biden and the US Congress.

Zelensky said they told him: "We have a difficult situation, the enemy is increasing their numbers. Our guys are braver but we need more weapons."

Ukrainians have endured intensifying Russian attacks on the country's infrastructure over the past two months.

With severe winter conditions setting in, missile and drone strikes have plunged cities around the country into darkness, and severed water and heat supplies to millions of Ukrainians.

The United States and European allies have rushed generators to the country in support, as well as extensive air defense systems needed to shoot down incoming rockets.

Patriot missiles could also defend Ukraine against short-range ballistic missiles, which Russia is seeking from Iran for the war, according to Western officials.