SCRANTON, Pa. (AP) — Under extraordinarily tight security, Ukrainian President Volodymyr Zelenskyy on Sunday visited the Pennsylvania ammunition factory that is producing one of the most critically needed munitions for his country's fight to fend off Russian ground forces.

Rep. Matt Cartwright, a Democrat who was among those who met with Zelenskyy at the Scranton Army Ammunition Plant, said the president had a simple message: “Thank you. And we need more.”

The Scranton plant is one of the few facilities in the country to manufacture 155 mm artillery shells and has increased production over the past year. Ukraine has already received more than 3 million of them from the U.S.

Zelenskyy's visit kicked off a busy week in the United States as he works tirelessly to shore up support for Ukraine in the war. He will speak at the U.N. General Assembly annual gathering in New York on Tuesday and Wednesday and then travel to Washington for talks on Thursday with President Joe Biden and Vice President Kamala Harris.

The area around the ammunition plant had been sealed off since Sunday morning, with municipal garbage trucks positioned across several roadblocks and a very heavy presence of city, regional and state police, including troopers on horseback.

As Zelenskyy’s large motorcade made its way to the ammunition plant in the afternoon, a small contingent of supporters waving Ukrainian flags assembled nearby to show their appreciation for his visit.

“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 those who greeted Zelenskyy's motorcade. “And I strongly feel that way.”

She said many of her friends’ parents have worked in the ammunition plant, and she called Zelenskyy’s visit “a wonderful thing.”

Laryssa Salak, 60, whose parents also immigrated from Ukraine, also said she was pleased Zelenskyy came to thank the workers. She said it upsets her that funding for Ukraine’s defense has divided Americans and that even some of her friends oppose the support, saying the money should go to help Americans instead.

“But they don’t understand that that money does not directly go to Ukraine, 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.”

The 155 mm shells made in the Scranton plant are used in howitzer systems, which are towed large guns with long barrels that can fire at various angles. Howitzers can strike targets up to 15 miles to 20 miles (24 kilometers to 32 kilometers) away and are highly valued by ground forces to take out enemy targets from a protected distance.

With the war now well into its third year, Zelenskyy has been pushing the U.S. for permission to use longer range missile systems to fire deeper inside of Russia.

So far he has not persuaded the Pentagon or White House to loosen those restrictions. The Defense Department has emphasized that Ukraine can already hit Moscow with Ukrainian-produced drones, and there is hesitation on the strategic implications of a U.S.-made missile potentially striking the Russian capital.

Russian President Vladimir Putin has warned that Russia would be “at war” with the United States and its NATO allies if they allow Ukraine to use the long-range weapons.

At one point in the war, Ukraine was firing between 6,000 and 8,000 of the 155 mm shells per day. That rate started to deplete U.S. stockpiles and drew concern that the level on hand was not enough to sustain U.S. military needs if another major conventional war broke out, such as in a potential conflict over Taiwan.

In response the U.S. has invested in restarting production lines and is now manufacturing more than 40,000 155 mm rounds a month, with plans to hit 100,000 rounds a month.

Two of the Pentagon leaders who have pushed that increased production through — Doug Bush, assistant secretary of the Army for acquisition, logistics and technology and Bill LaPlante, the Pentagon's top weapons buyer — were expected to join Zelenskyy at the plant, as was Pennsylvania Gov. Josh Shapiro.

The 155 mm rounds are just one of the scores of ammunition, missile, air defense and advanced weapons systems the U.S. has provided Ukraine — everything from small arms bullets to advanced F-16 fighter jets. The U.S. has been the largest donor to Ukraine, providing more than $56 billion of the more than $106 billion NATO and partner countries have collected to aid in its defense.

Even though Ukraine is not a member of NATO, commitment to its defense is seen by many of the European nations as a must to keep Putin from further military aggression that could threaten bordering NATO-member countries and result in a much larger conflict.

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Copp reported from Washington.