“There will also be more equipment and other necessary supplies for the front,” Mr. Zelensky said in his evening address on Thursday, striking an unusually upbeat tone. Unlike his most recent speeches, which focused on the difficult situation at the front line, those remarks read like a list of successful agreements reached with Kyiv’s allies.
Ukraine also signed a security pact with Japan on Thursday and, at an international conference in Berlin this week, secured more than $15 billion in promised aid from Western partners to help rebuild the country.
The various commitments came too late to help Ukraine avoid losing ground in the northeast and having its power plants crushed by Russian missiles. And Ukrainian officials and experts said that while the new deals would secure military support for Ukraine in the medium term, they fell far short of the level of support Ukraine needs to win the war.
“In Ukraine, this is perceived with cautious optimism,” said Serhii Kuzan, the chairman of the Ukrainian Center for Security and Cooperation, an independent research group.
“We are sincerely grateful for any help, and if we receive all the necessary weapons and ammunition on time, it will give us a chance to stabilize the front line,” Mr. Kuzan said. “But this is not enough to talk about the liberation of the occupied territories and victory as such.”
Still, Ukrainian officials have been enthusiastic about the security agreement with the United States. It is one of 17 similar arrangements signed with Western allies in recent months, each meant to provide Ukraine with sufficient security assistance to deter further Russian assaults, including deliveries of key weapons, training of troops and intelligence sharing.
“Given the nature of U.S. resources and its central role in NATO,” the pact signed by Presidents Zelensky and Biden on Thursday “is the most important of all these agreements,” Mick Ryan, a retired Australian general and fellow at the Lowy Institute, a Sydney-based research group, said in an initial assessment of the security deal.
Andriy Yermak, the head of Mr. Zelensky’s office, said in a statement that the pact was “the strongest of all agreements” between Ukraine and the United States, because it commits Washington to providing “a stable level of assistance to Ukraine every year for 10 years, covering key aspects of assistance, including defense.”
And while the agreement did not include concrete assurances of funding from the United States, Mr. Yermak said its commitments differentiated the pact from the 1994 Budapest Memorandum, which many Ukrainians say has failed to protect their country.
Under that memorandum, Ukraine agreed to return to Russia old Soviet nuclear weapons based on its territory in exchange from security guarantees from Russia, the United States and Britain. But the agreement did not detail what the security guarantees entailed, and included no promise of military assistance in the event of an attack. Ukrainian officials say that gave Russia free rein to attack their country, as it did starting in 2014.
Whether the new security agreement will fundamentally change the situation is unclear. The pact contains only pledges to work with Congress to secure additional military assistance, a task that may prove arduous given the Republican objections that delayed the passage of a multibillion-dollar arms package for months earlier this year.
What’s more, the new agreement can be terminated by either party, meaning that former President Donald J. Trump, who has repeatedly expressed his opposition to continued support for Ukraine, could abandon the deal if he defeats Mr. Biden in November’s election and returns to office next year.
In Kyiv, the Ukrainian capital, residents expressed skepticism about the various deals signed between Ukraine and its Western allies.
“It’s not clear exactly what the security agreements will give us, when they will come into effect, or if it’s just a communication effort,” said one resident, Ihor Kalashnyk. “Overall, what do they mean? What do they provide? No one understands.”
Mr. Biden said the deal put Ukraine on the road to NATO membership. But he has consistently opposed admitting the country to the military alliance while the war is ongoing, fearing that doing so could one day force American troops into direct combat with the Russian military.
The most pressing issue for Ukraine remains securing additional arms supplies. Mr. Zelensky said on Thursday that a recently approved multibillion-dollar American military aid package would allow Ukraine to equip more reserves to replace exhausted troops on the front line.
New ammunition from the package has already reached Ukrainian troops, soldiers and commanders have said in recent interviews, helping stabilize their defenses. Moscow has for months benefited from an advantage in the number of shells it fires, sometimes reaching a ratio of 10 Russian shells fired for every Ukrainian shell.
To supply its own troops as the war stretches further into its third year, Russia has increasingly relied on weapons shipments from allies like Iran and North Korea. South Korea’s defense minister told Bloomberg on Friday that North Korea had sent containers to Russia that could hold up to 4.8 million artillery shells.