Russia has enough troops in place to seize Kyiv or another Ukrainian city but not yet sufficient numbers for a full takeover and occupation of the country, Ukraine’s former defence minister has said, as Washington warned that an invasion could take place at any time.

Andriy Zagorodnyuk said in an interview with the Guardian that the situation looked “pretty dire”. “Russia could now seize any city in Ukraine. But we still don’t see the 200,000 troops needed for a full-scale invasion,” he said.

His comments follow ominous briefings by the Biden administration about the Kremlin’s military buildup on Ukraine’s border. The White House believes Moscow has assembled at least 70% of the firepower it needs to give Vladimir Putin the option of a major military operation by mid-February.

The US national security adviser, Jake Sullivan, said on Sunday that an invasion could take place at any time. “It could happen as soon as tomorrow or it could take some weeks yet,” he told NBC. “We’re in the window where something could happen. We believe the Russians have put in place the capabilities to mount a significant operation.”

He insisted, however, that Joe Biden was still pursuing diplomacy. “He’s reinforced and reassured our partners on the eastern flank. He’s provided material support to the Ukrainians, and he’s offered the Russians a diplomatic path if that’s what they choose instead, but either way, we are ready, our allies are ready,” he said.

“We’re prepared to sit down with the Russians, alongside our allies in Nato and other partners in Europe, to talk about issues of mutual concern in European security. And yes, that includes the placement of certain range systems of missiles.”

Later on Sunday, the White House reported that Biden had spoken on the phone to Emmanuel Macron before the French president’s visit to Moscow on Monday. “The leaders discussed ongoing diplomatic and deterrence efforts in response to Russia’s continued military buildup on Ukraine’s borders, and affirmed their support for Ukraine’s sovereignty and territorial integrity,” it said in a statement.

Before leaving Paris, Macron told the Journal du Dimanche newspaper that Russia’s objective was “not Ukraine, but a clarification of the rules … with Nato and the EU”.

He said his dialogue with Putin would probably be enough to prevent military conflict breaking out and that he believed the Russian president would be open to discussing wider issues.

US officials have warned that a full attack could lead to the swift capture of Kyiv and potentially result in 50,000 civilians killed or wounded, as well as up to 25,000 dead Ukrainian soldiers and 10,000 Russian ones. Millions could flee in a refugee crisis for Europe, they suggested.

They said the Russian army had now positioned 83 “battalion tactical groups” near Ukraine, each with between 750 and 1,000 soldiers. The figure has risen from 60 battalion groups two weeks ago, they added.

Zagorodnyuk said he did not believe a Russian invasion was inevitable. He said the remorseless Russian troop buildup was proceeding along textbook lines, but that the Kremlin’s intentions and strategy remained opaque.

“We don’t see a political endgame here,” he said. “If Putin seizes Kyiv, there will be full-scale war. The Ukrainian army forces will fight. There will be enormous resistance for all time. Why would you do that?

“Ukraine is not going to say: ‘Let’s join Russia.’ This is understood. Unless, of course, Putin is totally delusional and has his own understanding of reality. There will be blood, sanctions. Nobody needs that kind of international war in Europe right now.”

The Ukrainian foreign minister, Dmytro Kuleba, sought to play down “apocalyptic predictions” about an imminent Russian invasion, saying his country was strong and had unprecedented international support.

Zagorodnyuk said the US administration was right to release intelligence about Russian plans and capabilities. On Thursday, US officials claimed to have evidence of an elaborate plot by the Kremlin to make a “very graphic” fake video of a Ukrainian attack as a pretext for a military invasion.

Downing Street said on Friday it had “high confidence” that Russia was planning to fabricate a reason for attacking Ukraine. Zagorodnyuk said the plot seemed “a bit exotic”, but added that Moscow had previously carried out similar “false flag” operations.

The latest US assessment was shared late last week with House and Senate representatives in closed briefings. Putin had not made a final decision to invade, US intelligence analysts indicated. But they pointed to satellite images and communications among Russian forces that showed Moscow was in a position to carry out the largest military operation on land in Europe since 1945.

From Thursday, Russia will stage major military exercises with Belarus, within striking distance of Kyiv. According to the Nato secretary general, Jens Stoltenberg, Russia has deployed 30,000 combat troops, elite Spetsnaz units, Su-35 fighter jets and S-400 missile defence systems.

In total, there are 135,000 Russian troops on Ukraine’s border. They are stationed close to the Donbas region in the east, where the Ukrainian army has fought a low-intensity conflict with pro-Moscow separatists for almost eight years.

Russia has moved troops into Crimea, in the south, which Putin annexed in 2014. Its forces have been positioned in a way that suggest Russia could invade from three sides, the US thinks.

US officials briefing on Saturday said 14 battalion tactical groups were on their way to the border from other parts of Russia. The US believes the Kremlin would want between 110 and 130 battalion tactical groups for use in a full-scale invasion, but also says Putin could decide on a more limited incursion.

Including support units, Russia could be aiming to have 150,000 troops in place for a full-blown military offensive, one US official said, adding that the buildup could reach that level in the next couple of weeks.

Hundreds of elite US troops were due to arrive in Poland on Sunday. The Biden administration is sending additional soldiers to Poland, Romania and Germany to bolster Nato’s eastern flank.

The Kremlin rubbished Washington’s latest briefing as “fake”. Dmitry Polyanskiy, Russia’s deputy ambassador to the United Nations, said it was “another masterpiece of US propaganda war”. “Unnamed officials, undisclosed sources, no evidence,” he tweeted, alongside the hashtag #KeepCalmAndBlameRussia.

Ukraine’s embattled pro-western president, Volodymyr Zelenskiy, has sought to play down fears of invasion. His aide, Mykhailo Podolyak, said on Sunday the chances of a diplomatic solution to the crisis were “substantially higher” than a Russian attack.

Podolyak said Russia had been conducting large-scale troop rotations, manoeuvres and weapon deployments on a regular basis “to ensure constant massive psychological pressure” on Kyiv. “For our intelligence service and our armed forces, this Russian activity comes as absolutely no surprise,” he said.

Zelenskiy made Zagorodnyuk defence minister after winning a landslide election victory in 2019. Zagorodnyuk later left government and now runs a thinktank, the Centre for Defence Strategies. “Russia has increased its troop numbers in Belarus. They are getting ready for a potential invasion. The main concern I have is for Kyiv,” he said.

In Washington, efforts in Congress to impose economic punishment on Russia also appear to be growing. John Barrasso, the Republican ranking member of the Senate foreign relations committee, said on Sunday that it was clear that Putin alone would make the decision to invade.

“He is a predator. He’s going to see what it costs him and what he gets,” Barrasso told Fox News Sunday. “And if we put biting sanctions in place right now, without delay before an attack, financial as well as making sure Ukraine has the weapons that they need to defend themselves with first-class weapons, then that will make Putin think twice.”

Ben Cardin, a Democratic senator for Maryland and a senior committee member, said there were differences with Republicans over the content and timing of the sanctions package, but that he expected a bill to be presented to the House of Representatives soon.

“The bill is just about complete. We have a few issues we’re trying to resolve, but from a substance point of view, it will contain the strongest possible sanctions, including financial sanctions and personal sanctions,” he said.