U.S. officials have said they have no information to corroborate media reports suggesting Russia is preparing to use a tactical nuclear weapon on or near the battlefield in Ukraine.
Laura Cooper, a deputy assistant secretary of defense, said she had seen only "open-source reports" suggesting Russia might be moving tactical nuclear weapons by rail.
She added that the U.S. military had not seen anything to change its own nuclear posture.
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White House spokeswoman Karine Jean-Pierre, asked on October 4 about the reports, said the United States takes nuclear weapons and nuclear saber-rattling very seriously.
But she added, "We have not seen any reason to adjust our own strategic nuclear posture, nor do we have any indication that Russia is preparing to imminently use nuclear weapons."
Russian President Vladimir Putin has escalated the seven-month war in Ukraine by ordering a "partial" military mobilization, pushing through an illegal annexation of four regions of Ukraine, and promising to defend Russian territory "with any means at our disposal," including nuclear weapons.
The U.S. officials' comments came after The Times newspaper reported on October 3 that Putin was set to carry out a nuclear test on Ukraine's border. The London-based newspaper said NATO had warned its members about the test.
NATO said it also had not observed changes in Russia's nuclear posture, according to an alliance official quoted by Reuters.
Asked about the reports, Kremlin spokesman Dmitry Peskov said Western media and politicians were "engaging in a lot of exercises in nuclear rhetoric right now," and Russia declined take part in it.
Cooper also said the Pentagon closely monitored Russia's nuclear forces, a core part of its mission since the Cold War, and was also closely monitoring the rate of munitions spent by Ukrainian forces.
"We're watching their ammunition consumption rates very closely to make sure they have what it takes to counterattack," she said.
Cooper expressed confidence that the additional HIMARS systems provided by Washington in a new aid package announced on October 4 will strengthen Ukraine's capabilities on the battlefield.
The United States is providing additional ammunition and investing in ammunition production as are allies and partners, while Russia is turning to North Korea and Iran for help, she said.
Commenting on whether Washington is considering providing long-range missiles to Ukraine, Cooper said the HIMARS currently allowed the Ukrainian forces to hit most targets on the battlefield.
"We think that [the HIMARS] can achieve most goals, including in Crimea," Cooper said.