WASHINGTON — America will send $400 million more in military aid to Ukraine, officials announced Thursday, amid concerns that financial assistance for the war against Russia could decline a bit if Republicans take control of Congress.

The aid comes as the vote counting from Tuesday’s election continues, with Republicans inching closer to a narrow House majority and with control of the Senate hinging on tight races in Arizona, Nevada and Georgia.

According to the Pentagon, the aid package will contain large amounts of ammunition and, for the first time, four highly mobile Avenger Air Defense Systems. 

Included is ammunition for the High Mobility Artillery Rocket Systems, known as HIMARS, which Ukraine has been successfully using in its counteroffensive against Russia.

There also will be Stinger missiles for the HAWK surface-to-air anti-aircraft system, 10,000 mortar rounds, thousands of artillery rounds for Howitzers, 400 grenade launchers, 100 Humvees, cold weather gear and 20 million rounds of ammunition for smaller, individual guns and rifles.

At the White House, national security adviser Jacob Sullivan said the new aid package would include important air defense contributions.

“This increased air defense will be critical for Ukraine as Russia continues to use cruise missiles and Iranian-made drones to attack critical civilian infrastructure,” Mr. Sullivan said.

A Pentagon spokeswoman, Sabrina Singh, told reporters the new addition of short-range Avenger systems will help Ukraine protect its troops against lethal drones, cruise missiles and attacks from helicopters. She said she did not know when the systems will get to Ukraine or how long the training will take.

The additional ammunition and air defense capabilities come as Russian troops began pulling out of the key Ukrainian city of Kherson, in a widening retreat that could mark a turning point in the war. 

Kherson is the only provincial capital that Moscow captured, and the Russian withdrawal could allow Ukraine to win back territory in the south that it had lost. 

Ukrainian officials acknowledged that Moscow’s forces had no choice but to flee Kherson but remained cautious, fearing an ambush.

President Zelensky has repeatedly pressed the United States and other allies for advanced air defense systems. Such systems have become increasingly important for Ukraine to defend itself against Russian aerial attacks on critical electricity and water infrastructure, particularly as winter approaches and the humanitarian impact is expected to worsen. 

Ukrainian officials have said 40 percent of the country’s energy infrastructure has been damaged in Russian attacks.

Including the latest aid, America has committed more than $18.6 billion in weapons and other equipment to Ukraine since Russia attacked on February 24. 

The new package of aid will be done under presidential drawdown authority, which allows the Pentagon to take weapons from its own stock and quickly ship them to Ukraine, officials said.

Some conservative Republicans, advocating an “America First” foreign policy, have called for cutting back assistance to Ukraine, and others who support backing Ukraine have called for greater scrutiny on the assistance. 

The House GOP leader, Congressman Kevin McCarthy — who could become speaker if Republicans win the House — warned last month that his party will not write a “blank check” for Ukraine if they gain the majority.

His comments reflect concerns by some who question the need for federal spending abroad at a time of record-high inflation at home.

When asked about ongoing aid to Ukraine, Ms. Singh said it has had bipartisan support.

“I think there is, in Congress, on both sides of the aisle, a commitment to Ukraine that we’re in this for the long haul,” she said. “So even with the midterms and the outcomes, I think that Ukraine will still see security assistance and support from the United States in their fight.”

On Wednesday, President Biden also expressed optimism that support will go on, even if Republicans take control of one or both houses of Congress.

“I hope we’ll continue this bipartisan approach of confronting Russia’s aggression in Ukraine,” Mr. Biden told reporters.