WASHINGTON — The White House has approved an additional $200 million in arms and equipment for Ukraine, administration officials said on Saturday, responding to urgent requests from President Volodymyr Zelensky for more aid to stave off the Russian invasion.

The latest arms package, which officials say includes Javelin antitank missiles and Stinger antiaircraft missiles, follows a $350 million arms package the Biden administration approved last month. Altogether, the administration has sent $1.2 billion in weapons to Ukraine in the past year, officials said.

The weapons come from existing U.S. military stockpiles in Europe and are flown to neighboring countries such as Poland and Romania, where they are shipped overland into western Ukraine. In less than a week at the beginning of the Russian assault, the United States and NATO pushed more than 17,000 antitank weapons, including Javelins, into the hands of Ukrainian commanders.

Russia has so far not attacked these shipments because its forces have been too busy fighting in other parts of Ukraine, Pentagon officials said. But on Saturday, according to the Interfax news agency, Russia’s deputy foreign minister, Sergei A. Ryabkov, warned that Moscow would start firing on such shipments, stirring fears of an escalation to the conflict.

The West has rejected Mr. Zelensky’s repeated pleas to enforce a no-fly zone over Ukraine and to provide Ukraine with Polish MIG-29 fighter jets out of fear of drawing the United States and NATO into a direct confrontation with Russia, a nuclear power.

Instead, the Biden administration and 14 other allies have sent antitank and antiaircraft weapons that analysts say have been effective at attacking Russian military might on the ground and in the air. Pentagon officials said they were also working closely with Eastern European nations, in particular, to provide more surface-to-air missile defenses to Ukraine, adding that such deliveries could be announced in the next few days.