Kasparov said he believes that Putin will not stop his aggression with Ukraine, an UNIAN correspondent reported.

"Ukraine must be defended, supported, and armed now. It may seem far away to you, but it is the front line of a war the United States and the rest of the free world is fighting whether it admits it or not," Kasparov said.

"Sanctions are important, but it was obvious six months ago they were not enough to deter Putin, and he must be deterred," he said.

Kasparov also called on the West to stop treating Putin as a person with whom they can negotiate.

"Stop treating Putin like any other leader who can be negotiated with in good faith. Stop legitimizing his brutal regime at the expense of the Russian people. The opposition movement [murdered Kremlin critic] Boris [Nemstov] and I believed in and that Boris died for should also be openly supported, the way the West championed the Soviet dissidents. Let the people of Russia know that they have allies abroad, the way Ronald Reagan told those of us behind the Iron Curtain that he knew it was our leaders, not us, who were his enemies.”

According to Kasparov, official opinion polls carried out in Russia do not correspond to reality, and Putin does not actually enjoy broad public support.

"If you are truly popular you can allow a free media and free elections – and your critics are not gunned down in the street," he said.

“Putin rebuilt a police state in Russia in full view of the outside world and now he is confident enough of his power to attempt to export that police state abroad. To Georgia, to Ukraine, to Moldova. Where next? He is testing NATO now and he will test it further. Putin also provides a role model for the rest of the world’s dictators and thugs by proudly defying the superior forces of the free world.”

Kasparov’s full testimony to the U.S. Senate committee can be found here.