President Obama spoke for 90 minutes with Russian President Vladimir Putin on Saturday in what appeared to have been a testy exchange reflecting an escalating battle of wills and growing international tension over Ukraine.

Obama expressed “deep concern” over Russia’s “violation of Ukrainian sovereignty and territorial integrity . . . [and] breach of international law,” the White House said.

He called on Putin to pull Russian forces, now reportedly spread across Crimea, back to their bases in the autonomous Ukrainian region, according to a White House statement, and made clear that Russian refusal would lead to suspension of U.S. participation in planning for the upcoming Group of Eight summit in Sochi, Russia, scheduled for June, and “greater political and economic isolation.”

Putin gave little ground, according to a Kremlin account of the telephone conversation. Calling the Ukraine situation “extraordinary,” he charged that Ukrainian “ultranationalists,” supported by the U.S.-backed interim government that took over last week in Kiev, were threatening “the lives and health of Russian citizens and the many compatriots” living in Crimea.

“In the case of any further spread of violence to Eastern Ukraine and Crimea,” a statement issued by Putin’s office said, “Russia retains the right to protect its interests and the Russian-speaking population of those areas.”

Obama and U.N. Secretary General Ban Ki-moon, who also placed a phone call to Putin, urged the Russian leader to open an immediate dialogue with Ukraine’s new leaders and permit international monitors to assess the situation on the ground.

Kremlin statements did not mention the proposal for monitors. Putin’s office said he told Ban that if the violence did not cease, Russia “would not be able to stay away and would resort to whatever measures are necessary in compliance with international law” to protect the Crimea’s Russian-speaking population.

Saturday night, Canada announced it was pulling its ambassador from Moscow over the crisis in Ukraine. A statement issued by Prime Minister Stephen Harper after an emergency Cabinet meeting said Canada also will boycott meetings ahead of the G-8 economic summit being chaired by Russia in June.

Reports from Crimea have described violence going both ways, as demonstrators from the area’s Russian-speaking majority have attacked activists backing the new government, and vice versa.

At an earlier emergency U.N. Security Council meeting, the second in as many days, Samantha Power, the U.S. ambassador to the United Nations, said the dispatch of international observers from the United Nations or the Organization for Security and Cooperation in Europe was the best way to “get the facts, monitor conduct and prevent any abuses.”

Ukraine’s U.N. ambassador, Yuriy Sergeyev, agreed. But in an impassioned address and in later remarks to reporters, he charged that Putin’s government, which on Saturday requested and got approval from the Russian parliament to use military force in Crimea, was being disingenuous. Russian troops, he said, “were already there” and had been deploying across the region for days.

Vitaly Churkin, Russia’s U.N. ambassador, offered to the council a dramatically different definition of the problem and how to resolve it. He blamed outside intervention from the West for “whipping up” demonstrations that led to last week’s ouster of Ukraine’s Russian-backed government.

Problems began, Churkin said, only after President Viktor Yanukovych decided to postpone steps toward Ukrainian membership in the European Union.

“Why did this problem have to result in street demonstrations?” Churkin asked. “Why [were the demonstrations] encouraged from abroad . . . by people from the E.U.?” They, and not Russia, he said, were responsible for “crude interventions in the internal sovereignty of Ukraine.”

“How to get out of this situation? We need cooler heads to prevail,” Churkin said. He repeated a call made by Russia last week to return to the terms of a Feb. 21 agreement between Yanukovych and the pro-Europe opposition that would have allowed him to stay in power until the end of the year at the head of a national unity government.

The new government, he warned, had been taken over by “radicals” whose “actions could lead to very different developments, which the Russian Federation is trying to avoid.”

Russia has also questioned the impartiality of Robert Serry, a senior U.N. official dispatched by Ban to Ukraine last week on a fact-finding mission. Serry said Saturday that he was unable to comply with Ban’s instructions to travel to Crimea. After getting in touch with the pro-Russian Crimean government, he said in a statement, he had “come to the conclusion that a visit to Crimea today is not possible.”

“In Crimea, I would have conveyed . . . a message for all to calm the situation down and to refrain from any actions that could further escalate an already-tense environment,” Serry said.

Meanwhile, E.U. foreign-policy chief Catherine Ashton called for an “extraordinary” meeting of the E.U.’s Foreign Affairs Council in Brussels on Monday.

As the administration has tried to coordinate its response to the crisis with European allies, British Foreign Secretary William Hague said via Twitter that he had summoned Russia’s ambassador to London for a meeting and that he had consulted by telephone with his German counterpart.

Although Ukraine’s interim government called for implementation of a 1994 security pact it signed with the United States, Britain and Russia — and Hague said Britain supported consultations under the terms of that agreement — U.S. officials gave no indication that a military response was being considered.

In the “Budapest Memorandum,” under which Ukraine transferred its Soviet-era nuclear weapons to Russia for disposal, signatories pledged to preserve Ukraine’s territorial integrity and submit any dispute to consultations among them.

Obama’s call to Putin came after a lengthy White House meeting of the president’s national security advisers. Defense Secretary Chuck Hagel telephoned his Russian counterpart, Sergei Shoigu, and Secretary of State John F. Kerry called interim Ukraine president Oleksandr Turchynov, U.S. officials said.

The White House statement about the Obama-Putin call said the United States had “consistently said that we recognize Russia’s deep historic and cultural ties to the Ukraine and the need to protect the rights of ethnic Russian and minority populations with Ukraine.”

“The Ukrainian government has made clear its commitment to protect the rights of all Ukrainians and to abide by Ukraine’s international commitments, and we will continue to urge them to do so,” the statement said.