(AFP) - Ukraine's president thanked "every American family" in an emotional but defiant address to the US Congress Wednesday aimed at countering growing skepticism over US military aid as Kyiv's war with Russia drags into the new year.

It was Volodymyr Zelensky's first overseas trip since Moscow launched the full-scale invasion on February 24, a ruthless offensive that has upended the global economy and led to widespread destruction and death.

"On this special Christmastime, I want to thank you, all of you. I thank every American family which cherishes the warmth of its home and wishes the same warmth to other people," he told lawmakers who had given him a lengthy standing ovation as he walked into the House chamber.

The conflict has begun to recede from the headlines in the United States, with no end to the fighting in sight and Republicans who take over the House of Representatives in January showing signs of weakening resolve.

In what was billed by US media as the most important speech of Zelensky's career, he told a joint meeting of the House and Senate his country would never surrender, adding: "Ukraine is alive and kicking."

But he urged lawmakers -- many of whom were dressed in the blue and yellow of the Ukraine flag -- to do more for his nation through the brutal winter months, pleading for more weapons and financial aid.

"We have artillery. Thank you. Is it enough? Honestly -- not really," he said in a frank moment that pierced what had been an upbeat atmosphere, with his words accompanied by several outbursts of spontaneous applause.

- 'Absolute victory' -

Discussing his visit Tuesday to the war's southeastern front line, Zelensky presented lawmakers with a Ukrainian battle flag signed by soldiers from the besieged city of Bakhmut.

The Washington visit, which came some 300 days after Russian hitmen parachuted into Kyiv in a failed attempt to assassinate Zelensky, underlined Ukraine's near-total reliance on Washington for its defense.

It coincided with President Joe Biden announcing a first Patriot missile defense system for Ukraine, which has been devastated by air strikes leading to widespread power outages as winter bites.

Zelensky thanked politicians from both parties but his appearance was also calculated to counter fatigue for the war, particularly among isolationist Republicans who have voiced doubts over the continued flow of tax dollars to Kyiv.

Congress is poised to pass $45 billion in additional military and economic aid to Ukraine this week as part of its $1.7 trillion annual funding bill, bringing total US assistance to over $100 billion.

But House Minority Leader Kevin McCarthy recently warned that Republicans -- a number of whom snubbed the speech -- could reject significant continued funding for Ukraine when they take over the lower chamber in January.

Just six percent of Republican-leaning voters believed Washington was doing too much for Ukraine in a Wall Street Journal poll in March, but that had risen to 48 percent by November.

Zelensky is not the first leader of a country at war to address US lawmakers in person: British prime minister Winston Churchill appeared before Congress in the thick of World War II, and leaders from Iraq and Afghanistan have given speeches more recently.

The Ukrainian leader acknowledged the burden of history weighing on the United States, borrowing from 32nd president Franklin D Roosevelt as he told the Congress: "The American people in their righteous might will win through to absolute victory."

Earlier, Biden hosted Zelensky at the White House, telling his counterpart that Ukrainians "inspire the world."

© Agence France-Presse