During 27 months of war in the heart of Europe, President Biden has consistently resisted pressure from many of his allies to let Ukraine into NATO, convinced that it was the one step that could quickly result in American troops being sent into direct combat with Russia, a war he fears could escalate and even turn nuclear.

So on Thursday, he rolled out a new set of alternative steps, each designed to demonstrate to President Vladimir V. Putin of Russia, and to the Ukrainians, that the United States and its allies have no intention of packing up and leaving, as they did in Afghanistan, even if Ukraine remains outside NATO for years.

He signed a 10-year security pact — albeit one with vague commitments and an early exit option — with Ukraine’s president, Volodymyr Zelensky. Mr. Biden portrayed the agreement as a long-term guarantee of continued arms, intelligence support, advice and technology to win the current war and deter a new one.

And he said the United States would take the lead in providing a loan of about $50 billion to Ukraine to rebuild its devastated ports and power plants, buy weapons and close its budget gap. The money is to be repaid from interest generated from $300 billion in assets that Mr. Putin, inexplicably, left in Western financial institutions before his February 2022 invasion.

“Our goal is to strengthen Ukraine’s credible defense and deterrence capabilities for the long term,” Mr. Biden said moments after he and Mr. Zelensky signed the accord on the southwest coast of Italy, where the Group of 7 industrialized nations opened their annual leaders conference.