U.S. Secretary John Kerry on Sunday threatened a range of economic sanctions against Russia if it did not roll back its military takeover of Crimea, a strategic peninsula belonging to Ukraine.

Calling the Russian move "an incredible act of aggression really a stunning, willful choice by President Putin to invade another country," Kerry said in an interview on CBS News' Face the Nation that the United States and its major allies were prepared to impose "visa bans, asset freezes [and] isolation with respect to trade, investment" on Russia unless it reversed course.

"You just don't in the 21st century behave in 19th century fashion by invading another country on completely trumped up pretext," the secretary said.

Asked about President Barack Obama's 90-minute phone call with Putin, which the White House was calling the "toughest phone call of [Obama's] presidency," Kerry sought to portray his boss as a forceful crisis manager.

"The president was very strong," Kerry said, adding that Obama "asked Mr. Putin in fact told Mr. Putin it was imperative to find a different path, to roll back this invasion." Obama is engaged in marathon talks with foreign leaders over how to respond to Russia's moves in Crimea, which came after Putin won parliamentary backing to use military force in defense of Russian citizens in Ukraine. Most of Crimea's 2 million-plus citizens are ethnic Russians, and many demonstrated support for Russia's takeover of the peninsula, which had been under Soviet sovereignty before it was transferred to the control of Ukraine, then a Soviet satellite, in the 1950s. .

Kerry urged Putin to seek diplomatic solutions to his concerns in Ukraine, saying, "We don't want to return to the Cold War." Still, the secretary issued repeated threats of "repercussions" if Russia didn't lay off Crimea and Ukraine. "American businesses may well want to start thinking twice whether they want to do business with a country that behaves like this," he said.

Meanwhile, a Western official, said, "This is probably the most dangerous situation in Europe since the Soviet invasion of Czechoslovakia in 1968. "Realistically, we have to assume the Crimea is in Russian hands. The challenge now is to deter Russia from taking over the Russian-speaking east of Ukraine."

Ukraine has appealed for help to both NATO and, specifically, to Britain and the United States, both of which were co-signatories - with Moscow - to a 1994 accord guaranteeing Ukraine's security after the breakup of the Soviet Union.

Western countries are scrambling to respond to developments in Ukraine's Crimea, where Russian President Vladimir Putin has claimed the authority to deploy Russian troops in the biggest confrontation between Moscow and the West since the Cold War.

NATO ambassadors met in Brussels on Sunday to discuss their next steps, with Secretary General Anders Fogh Rasmussen accusing Russia of threatening peace and security in Europe.

"What Russia is doing now in Ukraine violates the principles of the United Nations Charter. It threatens peace and security in Europe. Russia must stop its military activities and its threats," Rasmussen said ahead of the meeting.

German Foreign Minister Frank-Walter Steinmeier said that the situation in Ukraine was "extremely dangerous" and he urged Russia to desist from transgressing on Ukrainian sovereignty and territorial integrity. Steinmeier also said Russia had no right to deploy its military in Ukrainian sovereign territory beyond the rules of the treaty on the Russian Black Sea fleet.

Britain will suspend its participation in preparations for a G8 meeting in Sochi after Russia violated Ukraine's sovereignty and territorial integrity, British Foreign Minister William Hague said on Sunday.

The United States has already said it will not take part in the meetings, and a source in President Francois Hollande's office said France had also pulled out.

U.S. Secretary of State John Kerry on Sunday condemned Russia's "incredible act of aggression" in Ukraine and threatened "very serious repercussions" from the United States and other countries, including sanctions to isolate Russia economically.

"You just don't in the 21st century behave in 19th century fashion by invading another country on a completely trumped up pretext," Kerry told the CBS program "Face the Nation."

Kerry added that Russia still has "a right set of choices" that can be made to defuse the crisis.

"It's an incredible act of aggression. It is really a stunning, willful choice by President Putin to invade another country. Russia is in violation of the sovereignty of Ukraine. Russia is in violation of its international obligations," Kerry added.

Kerry said U.S. President Barack Obama told Putin in a 90-minute phone call on Saturday that "there will be serious repercussions if this stands. The president ... told Mr. Putin that it was imperative to find a different path, to roll back this invasion and undo this act of invasion."

However, Russian sources said that Putin had told Obama that "there are real threats to the life and health of Russian citizens and compatriots on Ukrainian territory." Moscow reserved the right to intervene on behalf of Russian speakers anywhere they were threatened, Putin added.

Open gallery view U.S. Secretary of State John Kerry. Credit: AP