The site, which Sky News visited in 2023, produces critically needed munitions for his country's fight against Russia.

Democratic representative Matt Cartwright, who was among those meeting Mr Zelenskyy at the Scranton Army Ammunition Plant, said that the Ukrainian leader had a simple message: "Thank you. And we need more."

Mr Zelenskyy faces a busy week in the US as he works to shore up support for Ukraine's fight against Russia.

He is due to speak at the UN General Assembly annual gathering in New York on Tuesday and Wednesday, and travel to Washington on Thursday for talks with President Joe Biden and vice president Kamala Harris.

Recent weeks have seen Mr Zelenskyy pushing the US, and other Western allies, for permission to use longer-range missile systems to strike deeper into Russia.

So far the Pentagon and White House have not green-lit the loosening of restrictions. There has been hesitation due to the possibility of a US-made missile hitting Moscow, which could escalate the war.

Mr Zelenskyy will also present his "victory plan" to the US - of which the long-range weapons systems are said to be key.

But his trip comes at a key juncture for his country, as a victory for Donald Trump in the 5 November US presidential elections could prompt a reset of Washington's policy on Ukraine.

During a TV debate earlier this month, Mr Trump refused to say if he wanted Ukraine to defeat Russia and said he would try to end the war before taking office if he wins.

The area around the plant was sealed off ahead of Mr Zelenskyy's trip on Sunday, with rubbish trucks positioned as roadblocks and a heavy police presence visible.

As Mr Zelenskyy's motorcade entered the ammunition plant, a small contingent of supporters waved Ukrainian flags nearby.

"It's unfortunate that we need a plant like this, but it's here, and it's here to protect the world," said Vera Kowal Krewson, a first-generation Ukrainian American who was among the small crowd. "And I strongly feel that way."

Laryssa Salak, 60, whose parents also immigrated from Ukraine, said she was pleased Mr Zelenskyy came to thank the workers.

But she said it upset her that funding for Ukraine's defence against Russia divided Americans, and that even some of her friends opposed the support, saying the money should instead go to help Americans.

"But they don't understand that that money does not directly go to Ukraine," Ms Salak said. "It goes to American factories that manufacture, like here, like the ammunition. So that money goes to American workers as well. And a lot of people don't understand that."

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The Scranton plant is one of the few facilities in the US that manufactures 155mm artillery shells and it has stepped up production in the last year.

These are used in howitzer systems, which are highly valued by ground forces in taking out enemy targets from a distance.

At one point in the war, Ukraine was firing between 6,000-8,000 155mm shells per day.

The rate started to deplete US stockpiles, prompting concern.

In response, the US invested in restarting production lines and now manufactures more than 40,000 155mm shells a month - with plans to hit 100,000 a month.