President Biden walks to board Air Force One on Thursday at Andrews Air Force Base in Maryland en route to Portland, Ore. Gemunu Amarasinghe/Associated Press

WASHINGTON — President Biden pledged an additional $1.3 billion Thursday for new weapons and economic assistance to help Ukraine in its strong but increasingly difficult battle against the Russian invasion, and he promised to seek much more from Congress to keep the guns, ammunition and cash flowing.

The latest military aid, Biden said, will be sent “directly to the front lines of freedom.”

“Putin is banking on us losing interest,” Biden said. The Russian president is betting that “Western unity will crack … and once again we’re going to prove him wrong.”

The new package includes $800 million in military aid for much-needed heavy artillery, 144,000 rounds of ammunition and drones for the escalating battle in the Donbas region of eastern Ukraine. It builds on roughly $2.6 billion in military assistance that Biden previously approved.

There’s also a fresh $500 million in direct economic assistance to Ukraine for government salaries, pensions and other programs. That raises the total U.S. economic support to $1 billion since Russia’s invasion began nearly two months ago.

Ukrainian President Volodymyr Zelensky expressed his gratitude to the United States in his nightly address in Ukraine, saying the military aid was “just what we were waiting for.” Earlier in a virtual address to the World Bank meeting in Washington, he said his nation will also need up to $7 billion each month to make up for economic losses.

With tens of thousands of buildings damaged and key infrastructure in ruins, Ukraine “will need hundreds of billions of dollars to rebuild,” Zelensky said.

Biden underscored a need for the United States and Western allies to remain resolved in their support for Ukraine amid signs that Americans may becoming more wary of the war.

A poll published Thursday by The Associated Press-NORC Center for Public Affairs Research shows Americans’ desire to get involved has waned somewhat. Some 32 percent say the U.S. should have a major role in the conflict. That’s ticked down from 40 percent last month, though it’s slightly higher than the 26 percent who said so in February. An additional 49 percent say the U.S. should have a minor role.

The president also announced that Russian-affiliated ships would be barred from U.S. ports, though that appeared to be largely symbolic. Russian ships bring a tiny amount of the cargo unloaded in the U.S., and “my guess is that … a decent chunk of that was tankers transporting Russian oil which is now banned anyway,’’ said Colin Grabow, a research fellow who studies trade at the Cato Institute.

Overall, Biden said that $6.5 billion in security assistance that Congress approved last month as part of a $13.6 billion package for Ukraine could soon be “exhausted.” With the latest announcement, Biden has approved about $3.4 billion in military aid since Feb. 24. Congress’ overall total also included about $6.8 billion in direct economic assistance to care for refugees and provide economic aid to allies in the region impacted by the war – and additional funding for federal agencies to enforce economic sanctions against Russia and protect against cyber threats.

“Next week, I’m going to have to be sending to Congress a supplemental budget request to keep weapons and ammunition deployed without interruption,” Biden said.

Congress has signaled it is receptive to further requests and has been expecting there would be a need for further help for the Ukrainians. But the issue could become entwined with partisan fights over pandemic spending and immigration, complicating the pathway.

Senate Minority Leader Mitch McConnell, R-Ky., has expressed a willingness to consider more aid for Ukraine in recent weeks. “I think we need to say we want the Ukrainians to win, and we’re prepared to do everything we can to help them win,” McConnell said Monday in Shelbyville, Kentucky.

“We want to do more,” House Speaker Nancy Pelosi, D-Calif., told reporters at the Capitol, during an appearance with Ukrainian Prime Minister Denys Shmyhal. She said lawmakers would learn more about Biden’s latest funding request “in the next day or so, to be taken up as soon as we can. Next week.”

Biden has chosen retired Lt. Gen. Terry Wolff, a National Security Council official during the Obama administration, to oversee coordination of the security assistance to Ukraine, according to a White House official. The official, who spoke on the condition of anonymity to discuss the yet to be formally announced appointment, said Wolff was brought to the White House National Security Council team in recent days.

Biden spoke on the new assistance, and more broadly about the situation in Ukraine, hours after Russian President Vladimir Putin claimed victory in the strategic city of Mariupol. Putin, however, ordered his troops not to risk more losses by storming the last pocket of Ukrainian resistance in the war’s iconic battleground.

Biden in an exchange with reporters following his remarks called Russia’s claim on Mariupol “questionable.”

Russian forces have destroyed much of southeastern port city, which has witnessed some of the fiercest fighting of the war. By Russian estimates, about 2,000 Ukrainian forces remain holed up in a sprawling steel plant, even as Russia continues to pound the industrial site and issue ultimatums for surrender.

Biden sought to make clear to Russians that plenty more military assistance for Ukraine would be coming.

“Sometimes we will speak softly and carry a large Javelin, because we’re sending a lot of those,” Biden said, paraphrasing Theodore Roosevelt and referring to an anti-tank missile system.

The new U.S. military assistance is to include 72 155mm howitzers, 144,000 artillery rounds, 72 vehicles used to tow to the howitzers onto battlefields, and over 121 Phoenix Ghost tactical drones, as well as field equipment and spare parts.

Pentagon press secretary John Kirby said the drones were developed by the Air Force and fit a specific need of Ukrainian forces in the battle for the Donbas. The drones are produced by a U.S. company, Aevex Aerospace, in a program that began before Russia invaded on Feb. 24. Kirby said the Phoenix Ghost is similar to the armed Switchblade kamikaze drones that the Pentagon already is providing to Ukraine.

The 72 howitzers are in addition to the 18 announced last week the U.S. was transferring to Ukraine.

Biden’s decision to quadruple the number pledged in an arms package announced just last week reflects what is shaping up as a major ground battle in the contested Donbas region of eastern Ukraine.

The Russians have been deploying additional artillery there in recent days as they aim to expand their offensive and attempt to take full control of the Donbas after retreating from a failed attempt to capture Kyiv, the capital. Heavy weaponry like artillery is expected to play a key role in the fighting in the relatively confined area where Ukrainian and Russian-backed separatists have been battling since 2014.

The announcements on new aid came against the backdrop of International Monetary Fund and World Bank meetings this week dominated by how to manage the spillover from Russia’s war in Ukraine.

Ahead of his remarks, Biden met with Ukrainian Prime Minister Shmyhal. The Ukrainian leader also met on Thursday with Treasury Secretary Janet Yellen as well as Pelosi.

Yellen at a news conference said the economic aid the U.S. and allies have sent “is only the beginning of what Ukraine will need to rebuild.” She added that she’d be discussing efforts to further aid Ukraine with her Western counterparts this week.

“I think we stand united in recognizing we’ve got to find ways to meet Ukraine’s needs,” she said.

The Biden administration also announced a new program Thursday aimed at streamlining refugee applications for Ukrainians and others fleeing the fighting, while they are still in Europe. The U.S. will no longer routinely grant entry to those who show up at the U.S.-Mexico border seeking asylum, as thousands have.

The U.S. expects to admit up to 100,000 refugees from Ukraine and about 15,000 have come, mostly through Mexico.

Associated Press writers Ben Fox, Alan Fram, Fatima Hussein, Chris Megerian, Zeke Miller and Paul Wiseman contributed reporting.