Mrs. Merkel said that the West needs to take Mr. Putin's warning seriously.

"These statements should not be taken as empty talk, and taking them seriously is not a sign of weakness. On the contrary, it is a sign of political wisdom," former German Chancellor Angela Merkel said at the launch of the Chancellor Helmut Kohl Foundation in Berlin on September 27.

Her comments come after Russian President Vladimir Putin said last week that Moscow would use "all means to protect the country and its people" if its territorial integrity was threatened. Those using "nuclear blackmail" against Russia "should know that the wind can change direction," Putin said.

This is one of the rare times Ms Merkel has commented on Russia policy, since she left office last year. During her 16 years in power, Germany built good relations with Russia, but also relied heavily on its energy.

Former German Chancellor Angela Merkel at the launch event of the Chancellor Helmut Kohl Foundation in Berlin on September 27. Photo:AFP

She has also been criticized for her business ties with Moscow, despite supporting anti-Russia sanctions imposed by the West after Moscow annexed Crimea in 2014.

In June, Ms Merkel said her resignation last year could have influenced Mr Putin’s decision to launch a military operation in Ukraine. She also acknowledged her own failure to create “an additional negotiating framework with Russia on the European security order, in addition to the Normandy format”, which paved the way for the Minsk agreements of 2014 and 2015 to de-escalate the conflict in eastern Ukraine since 2014.

On September 21, Mr. Putin declared that Russia can use all available tools, including "weapons of different levels of destruction", to protect "territorial integrity." The "territorial integrity" mentioned by the Russian President may soon include four Ukrainian regions: Donetsk, Lugansk, Zaporizhia and Kherson, which have just held referendums on joining Russia.

Russian President Putin's warning has raised concerns among Western officials about the risk of Russia using small tactical nuclear weapons in the conflict in Ukraine. Ukrainian President Volodymyr Zelensky believes his Russian counterpart is not "making empty threats", while the European Union's foreign policy chief Josep Borrell said the warning must be taken "seriously".

US President Joe Biden accused the Russian President of disregarding his responsibility to prevent the proliferation of nuclear weapons. US National Security Advisor Jake Sullivan warned that Washington would respond decisively and impose "catastrophic" consequences on Russia if it used nuclear weapons in Ukraine.

Russia possesses the world's largest nuclear arsenal with about 6,000 nuclear warheads and a huge arsenal of intercontinental ballistic missiles (ICBMs).