US’s Antony Blinken and Ukraine's Foreign Minister Dmytro Kuleba hold a news conference in Washington. File/AP

The buildup of Russian troops near Ukraine has left US officials perplexed, muddying the Biden administration’s response.

Some Republican lawmakers have been pressing the US to step up military support for Ukraine. But that risks turning what may be mere muscle-flexing by Russian President Vladimir Putin into a full-blown confrontation that only adds to the peril for Ukraine and could trigger an energy crisis in Europe.

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The White House on Friday called on Russia to de-escalate tensions with Ukraine, saying it has raised its concerns about Moscow's military buildup directly.

"We also continue to have serious concerns about Russian military activities and harsh rhetoric towards Ukraine and call on Moscow to de-escalate tensions," White House spokesperson Jen Psaki told reporters.

Russian President Vladimir Putin attends a meeting. File photo

President Vladimir Putin has issued his own warnings. On Thursday, he said the West was taking Russia's directives not to cross its "red lines" too lightly.

But a weak US response carries its own risks. It could embolden Putin to take more aggressive steps against Ukraine as fears grow he could try to seize more of its territory. And it could cause more political damage for President Joe Biden at a time his popularity is dropping.

Knowing how to strike the right balance would be easier if the US had a better understanding of what Putin was trying to accomplish. But top officials admit they don’t know.

On Friday, the Kremlin said those remarks were a response to provocative actions by NATO including the arming of Ukraine. Ukraine objects to increasing Russian military activity near the 1208 mile (1944 km) border it shares with Russia.

US President Joe Biden speaks during a meeting. File photo

Despite a growing list of disputes, the Kremlin has maintained high-level contacts with Washington and spoken repeatedly of a possible summit between Putin and US President Joe Biden to follow up their initial meeting in Geneva in June, which Putin said had opened up room for an improvement in ties.

"We’re not sure exactly what Mr. Putin is up to,” Defense Secretary Lloyd Austin said Wednesday. A week earlier, Secretary of State Antony Blinken said, "We don’t have clarity into Moscow’s intentions, but we do know its playbook."