"In his (Putin's) megalomania, the rapid invasion of Ukraine has been halted by Ukraine's brave and determined actions," Christine Lambrecht told public broadcaster ZDF.

Earlier Sunday Putin ordered his defence chiefs to put the country's "deterrence forces" on high alert, accusing Western countries of taking "unfriendly" steps against his country amid Moscow's invasion of Ukraine.

"We would be well advised to take Putin and his declarations very seriously and not to underestimate him," said Lambrecht, calling on the West to remain "very vigilant".

The White House has already condemned Putin's announcement as "totally unacceptable" and an unnecessary escalation.

NATO chief Jens Stoltenberg also denounced it as "dangerous" and "irresponsible".