Vladimir Putin received treatment for an advanced cancer in April, Newsweek Magazine reported on Thursday, quoting three senior U.S. sources.
The sources who say they had seen an intelligence report presented to U.S. president Joe Biden, also told the magazine that the Russian president survived an assassination attempt in March.
"Putin's grip is strong but no longer absolute," one of the senior intelligence officers with direct access to the reports told Newsweek. "The jockeying inside the Kremlin has never been more intense during his rule, everyone sensing that the end is near."
All three sources cautioned however, that Putin's medical condition was increasingly harder to gage because the Russian leader has been isolating himself.
"Putin is unwell but whether he will die soon is only speculation," the sources told Newsweek adding that a vacuum in Russia's leadership can be very dangerous to the world.
They said intelligence agencies have carefully studied photographs of his meetings with visiting dignitaries and noted his insistence on maintaining a distance from them and avoiding any physical contact.
The sources referred to footage of Putin in a meeting with Sergei Shoigu on April 21, in which he appeared slouched in his chair with his right hand holding on to the desk before him.
Intelligence experts assessed his condition as grave and worse than previously believed.
Russian officials denied the president was ill.
"I don’t think there is a sane person who can suspect or see signs that he is sick," Foreign Minister Sergey Lavrov told French television last week.
The senior sources told Newsweek in response that Lavrov appear to be displaying loyalty to his leader more than making a medical assessment.