US Secretary of State John Kerry said Sunday that the US was willing to impose visa bans, asset freezes, trade and investment penalties on Russia, after President Vladimir Putin declared he had the right to invade Ukraine.

The top US diplomat added that Russia risked losing its position in the Group of Eight (G8), and that the organization's other global economic powers were willing to boycott an upcoming summit in Sochi scheduled for June.

Putin was granted permission by the Russian parliament on Saturday to use military force to protect Russian citizens in Ukraine. Russian forces have already seized key areas of the peninsula, where the country's Black Sea Fleet is based.

In a phone conversation with German Chancellor Angela Merkel Sunday evening, Putin said that Russian citizens and Russian-speakers in Crimea faced an "unflagging" threat from ultranationalists, and thus Moscow's actions were appropriate given the "extraordinary situation," according to a Kremlin statement.

Military standoff

Earlier on Sunday, hundreds of armed men believed to be Russian military surrounded a Ukrainian military base. Around two dozen Ukrainian soldiers placed a tank at the entrance to the facility, leading to a standoff, although no shots were fired.

In response, Ukraine's interim leadership ordered the country to put all armed forces on the highest alert and called up military reservists. Interim Prime Minister Arseniy Yatsenyuk warned Russia's actions put the two countries "on the brink of disaster."

"This is not a threat: this is actually the declaration of war to my country," he said in Kyiv.

Ukrainian authorities also launched a treason case against the head of the navy, who announced on Sunday he had switched allegiance to Crimea's pro-Russian regional leaders. In a televised statement, Denis Berezovsky said he "swears allegiance to the Autonomous Republic of Crimea" and told Ukrainian forces to lay down their arms.

'Act of aggression'

Speaking to US Sunday talk shows, Kerry said Putin should respect the democratic process that resulted in Kremlin-ally President Viktor Yanukovych's ouster late last month following weeks of heavy anti-government protests.

Kerry insisted Russia still had a "right set of choices" to end the crisis, otherwise G8 and other countries were prepared "to go to the hilt to isolate Russia," he told CBS' "Face the Nation." Along with the US and Russia, the G8 includes Britain, Canada, France, Germany, Italy and Japan.

Putin "is not going to have a Sochi G8, he may not even remain in the G8 if this continues. He may find himself with asset freezes on Russian business, American business may pull back, there may be a further tumble of the ruble," Kerry told NBC's "Meet the Press."

While Kerry derided what he called Russia's "brazen act of aggression," he stopped short of threatening US military involvement.

"The last thing anybody wants is a military option in this kind of situation. We want a peaceful resolution through the normal processes of international relations," he said.

Ahead of crisis talks with NATO ambassadors, the head of the 28-member alliance, Anders Fogh Rasmussen, warned Russia's actions threaten "peace and security in Europe."

dr/kms (AP, Reuters, AFP)