WASHINGTON - As the rest of the world watches Ukraine, so does the White House.

Amid reports of Russian military and paramilitary presence in Ukraine, President Obama spoke with Russian President Vladimir Putin for 90 minutes today, calling on Russia to withdraw its forces.

Obama "expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity, which is a breach of international law," according to a White House readout, warning that Russia's activity "would negatively impact Russia's standing in the international community" and "will lead to greater political and economic isolation," according to the White House's paraphrasing.

The U.S. will also stop participating in preparatory meetings for the G8 summit set to be held in Sochi June 4-5, the White House said.

At a meeting of the United Nations Security Council, U.S. Ambassador to the UN Samantha Power condemned Russia's reported advances into Ukraine, calling them "an act of aggression."

Earlier in the day, Obama's top national security officials gathered at the White House today to discuss U.S. options. That meeting included Defense Secretary Chuck Hagel, who spoke with his Russian counterpart by phone; Secretary of State John Kerry, who participated via video conference; Director of National Intelligence James Clapper; and Joint Chiefs of Staff Chairman Gen. Martin Dempsey.

The president did not attend that meeting, but he received a briefing on Ukraine from National Security Advisor Susan Rice, a senior administration official said.

On Capitol Hill, sanctions appear to be on the table: Sen. Bob Corker, R-Tenn., the top Republican on the Senate Foreign Relations Committee, said today that Congress will consider sanctions to freeze U.S. assets of Russian individuals and entities that have acted to undermine Ukrainian sovereignty. Such a move could be largely symbolic, unless Russian officials hold U.S. bank accounts or business interests.