The United States believes that Russian President Vladimir Putin's threatened retaliation against Ukraine over its drone attack last weekend has not happened yet in earnest and is likely to be a significant, multi-pronged strike, U.S. officials told Reuters.

The timing of the full Russian response was unclear, with one source saying it was expected within days. A second U.S. official said the retaliation was likely to include different kinds of air capabilities, including missiles and drones.

The officials spoke on condition of anonymity. They did not detail Russia's expected targets nor elaborate on intelligence matters. The first official said Moscow's attack would be "asymmetrical," meaning that its approach and targeting would not mirror Ukraine's strike last weekend against Russian warplanes.

Russia launched an intense missile and drone barrage at the Ukrainian capital Kyiv on Friday and Russia's Defense Ministry said the strike on military and military-related targets was in response to what it called Ukrainian "terrorist acts" against Russia. But the U.S. officials believe the complete Russian response is yet to come.

A Western diplomatic source said that while Russia's response may have started, it would likely intensify with strikes against symbolic Ukrainian targets like government buildings, in an effort to send a clear message to Kyiv.

Another, senior, Western diplomat anticipated a further devastating assault by Moscow. "It will be huge, vicious and unrelenting," the diplomat said. "But the Ukrainians are brave people."

The Russian and Ukrainian embassies in Washington and the White House did not immediately respond to requests for comment.

Michael Kofman, a Russia expert at the Carnegie Endowment for International Peace, said he expected Moscow might seek to punish Ukraine's domestic security agency, the SBU, for its role in last weekend's assault. To send a message, Russia could employ intermediate-range ballistic missiles for the attack, he said.

"Most likely, they will attempt to retaliate against (SBU) headquarters, or other regional intelligence administration buildings," Kofman said, adding Russia could also target Ukrainian defense manufacturing centers.

Still, Kofman suggested Russia's options for retaliation may be limited as it is already throwing a lot of its military might at Ukraine.

"In general, Russia's ability to substantially escalate strikes from what they are already doing - and attempting to do over the past month - is quite constrained," he said.

OPERATION 'SPIDER'S WEB'

Kyiv says Sunday's audacious attack employed 117 unmanned aerial vehicles launched deep from within Russian territory in an operation code-named "Spider's Web."

The United States assesses that up to 20 warplanes were hit - around half the number estimated by Ukrainian President Volodymyr Zelenskiy - and around 10 were destroyed.

The Russian government on Thursday denied that any planes were destroyed and said the damage would be repaired, but Russian military bloggers have spoken of loss or serious damage to about a dozen planes, including those capable of carrying nuclear weapons.

The strikes, prepared over 18 months and conducted by drones smuggled close to the bases in trucks, dealt a powerful symbolic blow to Russia, which throughout the Ukraine war has frequently reminded the world of its nuclear might.

Putin told President Donald Trump in a telephone conversation on Wednesday that Moscow would have to respond to attack, Trump said in a social media post.

Trump later told reporters that "it's probably not going to be pretty."

"I don't like it. I said: 'Don't do it. You shouldn't do it. You should stop it,'" Trump said of his conversation with Putin. "But, again, there's a lot of hatred."