WASHINGTON (AP) — Months into Russia's war in Ukraine, the United States had intelligence pointing to “highly sensitive, credible conversations inside the Kremlin” that President Vladimir Putin was seriously considering using nuclear weapons to avoid major battlefield losses, journalist Bob Woodward reported in his new book, “War.”

The U.S. intelligence pointed to a 50% chance that Putin would use tactical nukes if Ukrainian forces surrounded 30,000 Russian troops in the southern city of Kherson, the book says. Just months before, in the far northeast, Ukrainian troops had stunned the Russians by recapturing Kharkiv, Ukraine's second-largest city, and were pivoting to liberate Kherson, strategically located on the Dnieper River not far from the Black Sea.

National security adviser Jake Sullivan stared “with dread” at the intelligence assessment — described as coming from the best sources and methods — in late September 2022, seven months after Russia's invasion, the book says. It caused alarm across the Biden administration, moving the chance of Russia using nukes up from 5% to 10% to now 50%.

According to Woodward's account, President Joe Biden told Sullivan to "get on the line with the Russians. Tell them what we will do in response.”

He said to use language that was threatening but not too strong, the book says. Biden also reached out to Putin directly in a message, warning of the “catastrophic consequences” if Russia used nuclear weapons.

The famed Watergate reporter’s latest book also details Donald Trump's conversations with Putin since leaving office, Biden’s frustrations with Israeli Prime Minister Benjamin Netanyahu and more. The Associated Press obtained an early copy of Woodward's book, which is due out next week.

The book provides intriguing inside details about the U.S. assessment of the possibility of Putin deploying nukes, but the Biden administration’s concerns that Russia might use tactical nuclear weapons in Ukraine were not secret. From the president on down, numerous officials warned Putin against it.

Putin and other Kremlin voices also have frequently threatened the West with Russia's nuclear arsenal. In a strong, new warning to the West late last month, Putin said any nation’s conventional attack on Russia that is supported by a nuclear power will be considered a joint attack on his country.

The threat was aimed at discouraging the West from allowing Ukraine to strike deep into Russia with longer-range weapons and appears to significantly lower the threshold for the possible use of Russia’s nuclear arsenal. Biden has held off on allowing Ukraine to hit military targets deeper inside Russia with U.S.-provided missiles over fears of escalating the war, even as Ukrainian President Volodymyr Zelenskyy pleads for permission.

In another heated conversation laid out in Woodward’s book, Defense Secretary Lloyd Austin confronted his Russian counterpart, Sergei Shoigu, in October 2022.

“We know you are contemplating the use of tactical nuclear weapons in Ukraine,” Austin said, according to Woodward. “Any use of nuclear weapons on any scale against anybody would be seen by the United States and the world as a world-changing event. There is no scale of nuclear weapons that we could overlook or that the world could overlook.”

As Shoigu listened, Austin pressed on, noting that the U.S. had not given Ukraine certain weapons and had restricted the use of some of those it had provided. He warned that those constraints would be reconsidered. He also noted that China, India, Turkey and Israel would isolate Russia if it used nuclear weapons.

“I don’t take kindly to being threatened,” Shoigu responded, the book says.

“Mr. Minister,” Austin said. “I am the leader of the most powerful military in the history of the world. I don’t make threats.”

The Pentagon noted a call between Austin and Shoigu on Oct. 21, 2022. During the call, Austin “emphasized the importance of maintaining lines of communication amid the ongoing war against Ukraine,” according to a readout provided at the time.

U.S. intelligence officials saw China as having the most influence over Russia, and Biden called Chinese President Xi Jinping about the need for deterrence, Woodward wrote.

Xi agreed to warn Putin, according to the book. Biden and Xi met and agreed in November 2022 that “a nuclear war should never be fought” and noted their opposition to the use or threat to deploy nuclear weapons in Ukraine, a White House statement said at the time.

In terms of the war starting at all, the book details Biden’s criticism late last year of President Barack Obama’s handling of Russia seizing Crimea and a section of the Donbas in 2014, at a time when Biden was serving as the Democrat’s vice president.

“They f----- up in 2014,” Woodward wrote that Biden said to a close friend in December, blaming the lack of action for Putin’s aggression in Ukraine. “Barack never took Putin seriously.”

Biden was angry while speaking to the friend and said they “never should have let Putin just walk in there” in 2014 and that the U.S. “did nothing.”

White House spokeswoman Emilie Simons told reporters that “there are plenty of books that are being written about this administration and others" and that "we’re not going to comment on each anecdote that may come out of different reporting.”