FASANO, Italy — President Joe Biden and Ukrainian leader Volodymyr Zelenskyy signed a 10-year security agreement Thursday that they hailed as a milestone in relations between their countries, but that alone was not enough to stop Zelenskyy from wondering how much longer he could count on America’s support.

Zelenskyy also said his country “urgently” needed additional air defense systems to protect Ukrainians and the nation’s infrastructure from Russia’s continued bombardment.

The leaders signed the agreement on the sidelines of the annual Group of Seven summit, held this year in Italy, and Biden said the goal “is to strengthen Ukraine’s defense and deterrence capabilities.”

Zelenskyy said at a joint news conference that the signing made for a “truly historic day,” but he also wondered about the durability of support from the United States and other allies.

Ukraine’s president said the right question to ask is “for how long the unity in the world will remain? The unity in the U.S., together with European leaders” and how it will be influenced by the outcome of elections this year in many of those countries.

Topping that list is voting in the U.S. in November in a campaign that could see the return of Republican Donald Trump to the presidency. Trump has been skeptical of providing additional military aid to Ukraine, at one point criticizing the “endless flow of American treasure.” He more recently has expressed openness to lending money instead and has said Ukraine’s independence is important to the United States.

Biden said the U.S. has commitments from five countries that he did not name to provide Patriot missile and other air defense systems to Ukraine. He said countries that have been expecting the same weapons from the U.S. have been told they will have to wait because “everything we have is going to go to Ukraine until their needs are met.”

Zelenskyy said he “urgently” needed seven Patriot systems. Biden then told him, “You’ll have some relatively quickly.”

Germany is one of the five countries that have promised an additional Patriot system for Ukraine.

Zelenskyy went on to deliver a stark warning about Russian aggression, saying that “if Ukraine does not withstand, the democracy of many countries, I am sure, won’t withstand either.”

The U.S. and European countries also agreed to keep sanctioned Russian assets locked up until Moscow pays reparations for its invasion of Ukraine, clearing the way for a $50 billion loan package for Ukraine. Combined with new sanctions against Russia announced earlier in the week, Biden said the series of actions to support Ukraine show Putin that “he cannot wait us out. He cannot divide us.”

The highly anticipated agreement will leverage interest and income from more than $260 billion in frozen Russian assets, largely held in Europe, to secure a $50 billion loan from the U.S. and additional loans from other partners. Ukraine will receive the first payments sometime this year, a U.S. official said. The official spoke on the condition of anonymity to preview the agreement, which will be included in the G7 leaders’ statement on Friday.

Ukraine will be able to spend the money in several areas, including military, economic, humanitarian and reconstruction needs, the official said.

The leaders’ statement today will also preserve the option of confiscating the Russian assets entirely, for which the allies have yet to secure the political will, largely citing legal and financial stability concerns.