US President Barack Obama told his Russian counterpart, Vladimir Putin, in a 90-minute phone call Saturday that Russia had violated international law with its incursion into Ukraine and warned of US and allied reprisals.

The White House said Obama called on Putin to pull Russian troops back to their bases in Crimea and warned that continued violation of international law by Moscow would lead to further “political and economic isolation.”

Putin brushed off Obama’s warning, saying Russia has the right to “protect its interests and Russian-speaking populations” in Ukraine. He told Obama there was a “real threat weighing on the lives and the health of Russian citizens” in Ukraine, according to a Kremlin statement.

The call came as Obama’s national security team met to draw up US responses to the escalating crisis, after Russia’s parliament gave Putin the formal go ahead to send forces to Ukraine and officials in Kiev said 6,000 forces had already been sent to Crimea.

“President Obama expressed his deep concern over Russia’s clear violation of Ukrainian sovereignty and territorial integrity,” the White House said in a detailed statement about the call.

Obama told Putin that his actions were a “breach of international law, including Russia’s obligations under the UN Charter, and of its 1997 military basing agreement with Ukraine.”

“The United States calls on Russia to de-escalate tensions by withdrawing its forces back to bases in Crimea and to refrain from any interference elsewhere in Ukraine,” the statement said.

Obama also warned Putin that the people of Ukraine had the right to determine their own future.

“Russia’s continued violation of international law will lead to greater political and economic isolation,” the statement added.

Obama informed Putin, with whom he has had tense public interactions, that he would order US diplomats to stop preparatory meetings for the G8 summit in Sochi, Russia in June.

On Friday, US officials said Obama and other European leaders were unlikely to show up at the summit in the showcase Black Sea report if the Ukraine crisis was not resolved.

A man in unmarked uniform looks from atop of a military vehicle while standing guard as troops taking control the the Coast Guard offices in Balaklava, outskirts of Sevastopol, Ukraine, Saturday, March 1, 2014 (photo credit: AP/Andrew Lubimov)

A man in unmarked uniform looks from atop of a military vehicle while standing guard as troops taking control the the Coast Guard offices in Balaklava, outskirts of Sevastopol, Ukraine, Saturday, March 1, 2014 (photo credit: AP/Andrew Lubimov)

Obama did use the call to recognize that there were deep cultural ties between Ukraine and Russia and that there was a need to protect ethnic Russians in the former Soviet republic.

But he argued that the proper way to do that was through a dialogue with the government in Kiev and with international observers on the ground appointed by the United Nations Security Council and the Organization for Security and Co-operation in Europe (OSCE).

Obama also vowed to keep up consultations with the interim government appointed in Ukraine after the ouster pro-Russian president Viktor Yanucovych and bodies like the International Monetary Fund to support Kiev and to mitigate its deepening economic crisis.

Ukraine’s Prime Minister Arseny Yatsenyuk warned on Saturday evening that Russian intervention in Ukraine would constitute an act of war and would signal an end to relations between the countries.

British Foreign Secretary William Hague on Saturday condemned the Russian parliament’s decision to authorize military action and summoned the Russian ambassador to register his concerns.

“This action is a potentially grave threat to the sovereignty, independence and territorial integrity of Ukraine. We condemn any act of aggression against Ukraine,” Hague said in a statement issued by the Foreign Office.

The foreign secretary will visit Kiev on Sunday for talks with the interim Ukrainian government on the rapidly escalating situation in the Crimea.

The Kremlin said that Putin had not yet taken a decision on sending troops to Ukraine, after he was given the green light from parliament.

“I am deeply concerned at the escalation of tensions in Ukraine, and the decision of the Russian parliament to authorize military action on Ukrainian soil against the wishes of the Ukrainian government,” Hague said.

He said he spoke to Russian Foreign Minister Sergey Lavrov “to urge steps to calm this dangerous situation,” while the Foreign Office also summoned the Russian ambassador to London.

Russian Foreign Minister Sergei Lavrov, at UN Headquarters in Geneva, Switzerland, June 30 (photo credit:AP/Martial Trezzini, Keystone)

Russian Foreign Minister Sergey Lavrov, at UN Headquarters in Geneva, Switzerland, on June 30 (photo credit:AP/Martial Trezzini, Keystone)

The department’s political director Simon Gass later met Ambassador Alexander Yakovenko and “expressed deep concern at the Russian parliament’s decision to authorise military action in Ukraine against the wishes of the Ukrainian Government,” the Foreign Office said in a statement.

In a telephone call with German Foreign Minister Frank-Walter Steinmeier, Hague also agreed “on the need for international diplomatic action to address the crisis”.

EU foreign ministers will hold a new round of crisis talks on Ukraine in Brussels on Monday, when Hague said he would raise the issue of a further asset freeze.

During his visit to Kiev, Hague said he would discuss how Britain can help the Ukrainian government recover “improperly acquired assets”.

“The EU must agree urgently an asset freezing regime to target those suspected of laundering the proceeds of corruption,” Hague said, without naming names.

“On my instructions, the British embassy in Kiev has told the Ukrainian government that we stand ready to provide Ukraine with technical advice on asset recovery.”

Hague spoke to acting Ukrainian president Oleksandr Turchynov on Friday, offering Britain’s support and urging him to take steps to unify the crisis-hit country.

Meanwhile, NATO said it would calling talks Sunday on the escalating crisis in Ukraine.

Anders Fogh Rasmussen, who heads the North Atlantic Treaty Organisation, said on Twitter on Saturday that he was summoning its 28 ambassadors for emergency talks in Brussels on the “grave situation in Ukraine.”

NATO Secretary General Anders Fogh Rasmussen (photo credit: CC BY Wikipedia/World Economic Forum)

NATO Secretary General Anders Fogh Rasmussen (photo credit: CC BY Wikipedia/World Economic Forum)

“NATO Allies coordinate closely on grave sit in Ukraine. North Atlantic Council will meet tomorrow followed by NATO-Ukraine Commission,” he said.

Diplomatic sources said the ambassadors would meet at 1200 GMT with the Ukraine-NATO encounter expected three hours later at 1500 GMT.

“Urgent need for de-escalation in Crimea,” Rasmussen said. “Russia must respect Ukraine’s sovereignty, territorial integrity & borders, including with regard to movement of Russian forces in Ukraine.”

EU foreign policy chief Catherine Ashton urged Russia to refrain from deploying its armed forces in Ukraine and to abide by international law.

Any movement or stationing of troops should comply with international law and commitments, “notably under the UN Charter and the OSCE Final Act, the Budapest Memorandum of 1994 as well as bilateral treaties such as the one regulating the stationing of the Black Sea Fleet,” she said.