BERLIN (AFP) - Russian President Vladimir Putin "can be dangerous" and will always push other political leaders as far as he can, former United States secretary of state Hillary Clinton told a German newspaper on Sunday.

"A man like Putin always goes to the limits. He always tries to find out how strong the others are" and then repositions himself accordingly, she told Bild am Sonntag.

"His latest aggression in Ukraine must be answered with a joint reaction by the West," she said in an interview published to coincide with the Berlin launch of her book Hard Choices. "I believe he can be dangerous," she added, in response to questions about Moscow's annexation of Ukraine's Crimea Peninsula and Russia's tactics in the east of the former Soviet state.

Asked at her book presentation about what it was like to deal personally with the Russian strongman, she said he was "a tough customer, with a pretty thick skin".

Still, she agreed with a quip from one journalist that he sounded "like a Russian cowboy", to which she jokingly responded: "Yes, exactly - with or without shirt."