Nato’s chief said that, despite Russia’s threats about nuclear weapons, the alliance sees no need to change its own nuclear weapons alert level.

The alliance’s secretary-general, Jens Stoltenberg, spoke following talks on European security with Polish President Andrzej Duda at an air base in Lask, central Poland, where Nato’s Polish and US fighter jets F15 and F16 are based.

“We will always do what is needed to protect and defend our allies, but we don’t think there is any need now to change the alert levels of Nato’s nuclear forces,” Mr Stoltenberg said.

The Kremlin began an invasion of Ukraine last week and on Tuesday shelled the centre of the Ukrainian city of Kharkiv.

Russia has raised the spectre of nuclear war, reporting on Monday that its land, air and sea nuclear forces were on high alert following President Vladimir Putin’s weekend order.

Nato itself has no nuclear weapons, but three of its members do, the United States, Britain and France.

“We strongly believe it’s reckless and irresponsible the way Russia is speaking about nuclear weapons,” Mr Stoltenberg said, standing in one of the base’s hangars, which has been turned into a conference room with Polish and Nato flags and jets around it.

Mr Stoltenberg stressed that Russia has signed a number of deals agreeing that nuclear war cannot be won and should not be fought.

He called on Russia to “stop the attacks and withdraw all its forces and engage in good faith in diplomatic efforts” to end the war.

Government delegations from Ukraine and Russia met for talks in southeastern Belarus on Monday, but the outcome was not immediately clear.

“This is a horrendous, horrific invasion of an innocent country and we see that civilians are killed.

“It’s a brutality that has to stop immediately,” Mr Stoltenberg said.

The United States recently reinforced the eastern flank of Nato’s territory with some 5,000 additional troops deployed to Poland and Romania.

Both Nato countries border Ukraine, which is not part of Nato.

A number of French troops were travelling to Romania on Tuesday to further strengthen the region, Stoltenberg said.

The alliance is to discuss further reinforcement steps at its next summit in June in Madrid.

It is Nato’s responsibility to “ensure that we don’t see a development where a conflict in Ukraine spiralled out of control and becomes a full-fledged confrontation between Nato and Russia in Europe,” Mr Stoltenberg said.

He said alliance commanders are insisting that they are “able to maintain deconfliction (contacts) with Russia,” he said.

From Lask, Stoltenberg flew to Tallin, Estonia, for talks on security of the Baltic region which borders Russia and its ally Belarus.