As Russia’s foreign minister made clear that Moscow’s territorial ambitions extend beyond Ukraine’s eastern territories, the United States said on Wednesday that it would send four more advanced multiple-rocket launch vehicles to Ukraine.

The rocket launchers, which can unleash salvos that rival the devastating effect of an airstrike from a jet loaded with precision-guided bombs, are part of a raft of new longer-range weapons that the United States has been providing to the outgunned Ukrainian military. They underline Washington’s resolve to help counter Russian military might and President Vladimir V. Putin’s goal of subjugating Ukraine, a sovereign country.

But with Russia until recently making incremental but steady gains on the battlefield and with Ukraine’s Western allies struggling to keep up with Ukraine’s seemingly insatiable appetite for weapons, Ukraine faces an uphill struggle to gain battlefield parity. And there are disagreements between Ukrainian and American officials over what that would take.

Briefing reporters at the Pentagon on Wednesday, Defense Secretary Lloyd J. Austin III said the four more advanced multiple-rocket launch vehicles would bring the total number provided by the United States to 16.

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Mykhailo Podolyak, an adviser to President Volodymyr Zelensky of Ukraine, said in June that Ukraine needed 300 multiple-launch rocket systems and 500 tanks, among other things, to better compete with the forces Russia can field. That is several times more than what Ukraine has been promised.

Michael G. Vickers, the Pentagon’s former top civilian official for counterinsurgency strategy, said this month that the Ukrainians needed at least 60, and perhaps as many as 100, HIMARS or other multiple-launch rocket systems, to win the artillery battle.

Also looming over the weapons transfers is the question of how long American resolve will last amid high gasoline prices and increasing demands for national sacrifice in Europe and the United States. The Biden administration has also been hesitant to provide arms that could reach into Russian territory and, potentially, touch off a broader war.

The dozen M142 HIMARS — an acronym for High Mobility Artillery Rocket Systems — already provided from Pentagon stockpiles have already made a difference on the battlefield, Mr. Austin said. Ukrainian soldiers have used them to destroy Russian command-and-control centers and ammunition depots. Each M142 HIMARS truck carries six guided rockets loaded with 200 pounds of high explosives that can hit targets 50 miles away.

“That affects the tempo of the fight, and potentially creates some opportunities here,” Mr. Austin said. “There’s a lot more to be done — the HIMARS alone will not change, or win or lose a fight.”

Ukraine’s defense minister, Oleksii Reznikov, said this week Ukraine’s military needed at least 100 of the American launchers for them to “become a game changer on the battlefield.” The HIMARS and other rocket launchers already shipped to Ukraine have helped destroy about 30 Russian command stations and ammunition storage units, he said.

“This has significantly slowed down the Russian advance and dramatically decreased the intensity of their artillery shelling,” Mr. Reznikov said in an online interview on Tuesday for the Atlantic Council, a Washington research group. “So it’s working.”

As debate continues in the West over what is needed to rein in Russian forces, Russia’s foreign minister, Sergey V. Lavrov, said on Wednesday that Russia’s ambitions in Ukraine now stretched beyond the country’s eastern territories, a departure from the Kremlin’s earlier claims that it is not waging a war of imperial expansion.

As Ukraine has stepped up attacks on Russian forces in southern Ukraine in a possible prelude to a large-scale counteroffensive, Mr. Lavrov said that Moscow was also eyeing the Kherson and Zaporizka regions of Ukraine’s south, parts of which are occupied by the Russian forces, as well as “a number of other territories.”

Eric Schmitt contributed reporting.