U.S. President Barack Obama on Saturday threatened Russian President Vladimir Putin with political and economic isolation” over Russia’s military incursion in Ukraine.

AFP reported that the warning came in a 90-minute phone call between the two leaders. Obama reportedly told Putin that Russia had violated international law with its incursion into Ukraine.

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.”

The call came as Obama’s national security team met to draw up U.S. 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, according to AFP.

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 American diplomats to stop preparatory meetings for the G8 summit in Sochi, Russia in June.

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, reported AFP.

At the same time, 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).

Meanwhile on Saturday, acting Ukrainian President Olexander Turchynov placed the Ukrainian armed forces on “high alert” after Russian forces stormed the Crimean peninsula.

Turchynov reportedly ordered increased security at nuclear power plants, airports and other strategic infrastructure, because of the threat of “potential aggression” from the Russian troops.