U.S. Approved More Weapons for Ukraine. Now It’s a Race Against Time.

Last Sunday, as Russia put pressure on Ukrainian forces across a 600-mile front line, Ukraine received a shipment of anti-armor rockets, missiles and badly needed 155-millimeter artillery shells. It was the first installment from the $61 billion in military aid that President Biden approved just four days earlier.

A second batch of those weapons and ammunition arrived on Monday. And a fresh supply of Patriot interceptor missiles from Spain arrived in Poland on Tuesday. They would be at the Ukrainian front soon, a senior Spanish official said.

The push is on to move weapons to a depleted Ukrainian army that is back on its heels and desperate for aid. Over the last week, a flurry of planes, trains and trucks have arrived at NATO depots in Europe carrying ammunition and smaller weapon systems to be shipped across Ukraine’s borders.

“Now we need to move fast, and we are,” Mr. Biden said on April 24 when he signed the bill approving the aid. He added, “I’m making sure the shipments start right away.”

But it may prove difficult for Mr. Biden and other NATO allies to maintain the urgency. Weapons pledged by the United States, Britain and Germany — all of which have announced major new military support over the last three weeks — could take months to arrive in numbers substantial enough to bolster Ukraine’s defenses on the battlefield, officials said.

That has raised questions about Ukraine’s ability to hold off the Russian attacks that have had Kyiv at a disadvantage for several months.

Yet there is little time for Ukraine to lose against a steady Russian advance.

Avril D. Haines, the director of U.S. national intelligence, told Congress on Thursday that Russia could potentially break through some Ukrainian front lines in parts of the