The Biden administration on Sunday condemned Vladimir Putin’s decision to place Russia’s nuclear deterrence forces on high alert. The White House also faced growing calls from senior Republicans to target Russia’s energy sector with new sanctions.

As Russia’s invasion of Ukraine entered its fourth day, the US also expressed guarded optimism over talks between delegations from the two countries set to take place inside Ukraine, near the Belarusian border, on Monday.

Speaking on ABC’s This Week, the White House press secretary, Jen Psaki, described the nuclear deterrence announcement as an example of Putin “manufacturing threats that don’t exist in order to justify further aggression”.

In televised comments, Putin said he had ordered “the deterrence forces of the Russian army to a special mode of combat duty”, due to “aggressive statements” from Nato leaders. Analysts told the Guardian that while the order itself was not immediately clear, it was not indicative of preparation for a first strike.

Psaki said: “At no point has Russia been under threat from Nato, has Russia been under threat from Ukraine, this is all a pattern from President Putin. And we’re going to stand up for it. We have the ability to defend ourselves, but we also need to call out what we’re seeing here from President Putin.”

Biden administration officials expressed tentative support for planned talks between Russian and Ukrainian delegations, as announced by the Ukrainian president, Volodymyr Zelenskiy.

The US ambassador to the United Nations, Linda Thomas-Greenfield, told CNN’s State of the Union the US would “look forward to what comes out of those discussions.

“As you know … we leaned in on diplomacy with the Russians throughout this process and we hoped that Putin would find a way to the negotiating table and he made the unfortunate decision of aggression over diplomacy.”

Pressed on whether she believed the talks announcement indicated a good faith effort on behalf of Russia, Thomas Greenfield responded: “I can’t get into Putin’s head or into Russian reasoning, so it remains to be seen.”

The talks announcement was tentatively welcomed by the Nato secretary general, Jens Stoltenberg, who told CNN he had “absolute and full confidence” in Zelenskiy’s judgment on “whether it is right to sit down and find a political solution”.

But Stoltenberg also expressed concerns about Russia’s motivations.

“It remains to be seen whether Russia is really willing to make some serious compromises and also to respect the sovereignty of Ukraine,” he said.

Stoltenberg characterized Putin’s decision to order Russia’s nuclear deterrence forces on high alert as “dangerous rhetoric” and “a behaviour that is irresponsible”.

The Biden administration has issued tough sanctions, targeting banks and the finances of some Russian oligarchs as well as restricting export of vital technologies key to Russian military and economic development.

Over the weekend, the US and its European allies announced plans to target the Russian central bank’s foreign reserves and to block selected Russian financial institutions from the Swift messaging system for international payments.

But the sanctions have not yet targeted oil and gas exports, which reportedly accounted for 36% of Russia’s annual budget last year. That has lead to criticism both inside the Ukraine and in the US.

On Sunday Tom Cotton, a Republican senator from Arkansas and a prominent foreign policy hawk, urged the administration to continue to amplify sanctions.

“It’s time for the president and some of our European partners to quit pussyfooting around,” he told ABC. “The financial sanctions announced last night are riddled with loopholes.”

Cotton was also grilled on Donald Trump’s stance on the war. Trump, who often praised Putin while he was in the White House, finally condemned the invasion during a speech on Saturday night, but also continued to praise the Russian leader.

Cotton refused four times to condemn or comment on Trump’s record.

The Biden administration has not ruled out further sanctions and has alluded to further measures being taken as the war progresses.

“The purpose of the sanctions are to put as much pressure on the Russian economy as possible. And we want to do as much as we can to protect the impact on our own economy,” Thomas-Greenfield said.

“But we’re continuing to look at new and even harsher measures against the Russians.”