Despite all the assurances from Moscow, US President Joe Biden fears a Russian invasion of Ukraine in the next few days.

Biden said in Washington on Thursday that the threat of an invasion is "very high" and that he believes it could happen "in the next few days."

The United States had previously warned of a Russian provocation following reports of escalating fighting in eastern Ukraine.

"We have been saying for some time that the Russians could use such an approach to justify a military conflict," US Secretary of Defense Lloyd Austin said at the conclusion of a two-day NATO meeting in Brussels.

The reports of increasing shelling are therefore "disturbing", emphasized Austin.

Ukraine and the pro-Russian separatists in eastern Ukraine had previously accused each other of fueling the fighting in the conflict zone.

Meanwhile, the United States has received a response from Russia to the security guarantee proposals it submitted, according to a US State Department official.

The reply was delivered to the United States Ambassador to Russia, John Sullivan, in Moscow.

As reported by the Russian news agency RIA, the Russian government is demanding the complete withdrawal of US troops from Central and Eastern Europe.

Russia awaits specific proposals from the United States and NATO that NATO will not expand further east.

Arms control issues could not be considered separately from other issues.

A de-escalation requires that Ukraine adhere to the Minsk Agreement and no longer receive weapons.

Amid the Ukraine crisis, Russia expelled the deputy head of the American embassy in Moscow.

The expulsion of diplomat Bart Gorman was "without reason" and represents a "step of escalation," the State Department said in Washington.

"We'll check our answer."

Borrell: No one has any evidence of a deduction

Austin reiterated in Brussels on Thursday that the US has so far seen no signs of a Russian troop withdrawal.

On the contrary, Russia is even increasing the number of its 150,000 soldiers on the border with Ukraine.

In addition, Moscow is flying in additional military equipment and the operational readiness of the Russian forces in the Black Sea has been increased, the Pentagon chief emphasized.

London had also accused Russia of further increasing its troops.

EU foreign policy chief Josep Borrell also attended the NATO meeting.

He emphasized: "No one has any indication of a withdrawal of troops." Instead, there are signs of bombing in eastern Ukraine and an intensified Russian disinformation campaign.

When the time comes, the EU will not hesitate to "act decisively" and impose tough sanctions.

These mainly affect the energy sector, said Borrell.

Federal Defense Minister Christine Lambrecht (SPD) said on the sidelines of the meeting that she could not yet confirm any Russian troop withdrawal.