US President Joe Biden on Friday once again told his Russian counterpart Vladimir Putin to "take action" against ransomware hackers operating from Russia, but the Kremlin gave little sign of listening.

The White House said Biden and Putin spoke for about an hour by phone, focusing on the ransomware threat and separately about allowing humanitarian aid into Syria, where Russia is the main supporter of the Assad regime.

Talking to reporters, Biden said he "made it very clear" to Putin that "we expect them to act" against ransomware gangs.

It was the same message Biden delivered in person to Putin in their first summit meeting in Geneva in June.

The Kremlin sidestepped the issue, however, insisting after Friday's phone call that Washington had not even asked for assistance.

"Despite readiness from the Russian side to jointly clamp down on criminal activity in the informational sphere, over the past month no requests have been received from the relevant US departments," a statement from Putin's office said.

Told of the Kremlin's claim, a senior Biden administration official said there had been "multiple, specific requests for action."

The United States has "been clear to what Russia's responsibility is," said the official, speaking on condition of anonymity.

- Russia shielding hackers? - 
Ransomware attacks on the United States and other countries have increased sharply in the past year and have included a high-profile operation that shut down a major fuel pipeline in the eastern United States.

Other ransomware targets have included city police departments, hospitals and private companies.

US officials say the main geographical source of these attacks, where gangs force victims to pay ransom in exchange for lifting damaging or even crippling cyber intrusions, is Russia.

The US government already blames Russian spy agencies for organizing other huge cyber strikes and meddling in the tense last two presidential elections.

The White House says it does not believe the Kremlin itself is behind the ransomware incidents.

However, US officials say the Putin government is at minimum failing to clamp down on criminal gangs operating out of its own territory.

Biden repeated there would be consequences if nothing changes "when a ransomware operation is coming from his soil, even though it's not sponsored by the state."

He indicated that US officials might even be able to show Russia the identities of the assailants. "We expect them to act if we give them enough information to act on who that is," he said.

In its own statement after the call, the White House said Biden told Putin he would order "any necessary action" to defend Americans and critical infrastructure in the face of this continuing challenge."

The senior Biden administration official said the president "believes strongly" in the chances of progress through high level engagement with the Russians and that any real resolution will not be made for some time.

If it does come to US retaliation, some of that "may be manifest and visible, some may not be," the official said.