WASHINGTON — The United States will send an additional $800 million worth of military and other security aid to Ukraine and step up intelligence sharing, American officials announced on Wednesday, as Russian forces appeared to be preparing for a new offensive in the country’s east.

President Biden promised that the assistance would include more of the weapons that the United States has previously given Ukraine, as well as new capabilities. The expanded intelligence sharing will make available more information about Russian troops in Crimea and the eastern Donbas region, parts of the country that Russia and Russian-backed separatists have occupied since 2014 or 2015.

Mr. Biden delivered the news about the arms package to President Volodymyr Zelensky of Ukraine during a nearly hourlong telephone call on Wednesday, according to White House officials. In a statement, Mr. Biden vowed to continue efforts to support Ukraine, which has been pummeled by Russian artillery over the past month.

“As I assured President Zelensky, the American people will continue to stand with the brave Ukrainian people in their fight for freedom,” Mr. Biden said.

The issue of what military assistance to send Ukraine, and how fast to send it, has taken on greater urgency as the war has become a series of grinding battles between the Ukrainian and Russian armies. Mr. Zelensky has pleaded for more sophisticated weapons from the United States and its allies.

The Biden administration has responded with antitank weapons, including Javelin missiles, and other weapons that Ukrainian soldiers have used to repel Russian forces, especially in the north of the country. The United States has so far resisted helping Ukraine acquire fighter jets and other weaponry that could be used against targets inside Russia.

Mr. Biden said on Wednesday that the additional aid for Ukraine would include more sophisticated weaponry, such as artillery systems and armored personnel carriers.

Top Pentagon officials met with the leaders of the largest U.S. military contractors on Wednesday to discuss the flow of arms to Ukraine and what additional capabilities would help the country defend itself.

But the United States is also looking at advanced weapons in European stockpiles that Ukrainian troops already know how to use.

Mr. Biden said the United States would transfer additional helicopters as well, though he did not indicate whether they would come directly from the United States or from another country. The United States previously provided Ukraine with five Russian-designed Mi-17 helicopters that the Afghan military had once used.

The United States is also providing 18 155-millimeter howitzers, the first time it has sent that weapon to Ukraine, and 40,000 artillery rounds.

Last week, Slovakia agreed to send Ukraine a Soviet-era S-300 air defense system after the United States committed to positioning a more modern Patriot system in Slovakia. The United States has also helped European countries transfer Russian-designed T-72 tanks to Ukraine.

Russian forces failed to take Ukraine’s capital, Kyiv, after trying for several weeks and now appear to be regrouping in eastern Ukraine, including in the Donbas region. Western officials say they expect the Kremlin to mount a major offensive there.

Independent analysts have said that Russia hopes to destroy Ukraine’s army and then might attack Kyiv from the south. European intelligence officials said President Vladimir V. Putin of Russia wanted to make significant battlefield gains ahead of May 9, when he is set to deliver an annual speech commemorating the Soviet Union’s victory in World War II.

John F. Kirby, the chief Pentagon spokesman, said the latest weapons shipment approved by Mr. Biden would be sent to Ukraine as soon as possible.

“We’re aware of the clock,” he said at a news conference on Wednesday. “And we know time is not our friend.”

As the conflict in Ukraine has evolved, intelligence agencies have adjusted their approach to ensure officials have flexibility “to share detailed, timely intelligence with the Ukrainians,” a U.S. intelligence official said, speaking on the condition of anonymity to discuss the handling of classified material.

Biden administration officials have said they have been giving Ukraine the most relevant information at any given moment. Still, the administration has been reluctant to help the Ukrainians target Russian forces in Russia, and Republican lawmakers said that concern by the administration had extended to Russian forces in Crimea and the Donbas.

The stepped-up intelligence sharing was reported earlier by The Wall Street Journal.

U.S. officials have defended their intelligence sharing with Ukraine. On Tuesday, Kathleen H. Hicks, the deputy secretary of defense, said that “the intelligence support that we have provided has been vital.” And she said the information given to Ukraine had been “high-end.”

Other officials said that as the Russian military shifted its strategy away from the attack on Kyiv to reinforcing the operations in the Donbas, U.S. intelligence agencies began to look at whether their guidance on what information could be shared needed to be expanded.

Republicans have been critical of the Pentagon and the Office of the Director of National Intelligence, saying they have failed to provide enough information to Ukraine about the Russian forces and separatist groups that have been stationed in parts of Ukraine since 2014 or 2015.

In a letter released on Monday, Senate Republicans said they were concerned that not enough was being done to share critical intelligence with Ukrainians. The letter, from Senator Marco Rubio of Florida and others, specifically referred to providing intelligence to the Ukrainians to help them “retake every inch of Ukraine’s sovereign territory, which includes Crimea and the Donbas.”

Helene Cooper contributed reporting.