US President Barack Obama has told his Russian counterpart Vladimir Putin that Russia has flouted international law by sending troops to Ukraine.
The BBC reports that in a 90-minute telephone conversation, Mr Obama urged the Russian leader to pull forces back to bases in Crimea.
Mr Putin responded by saying that Moscow reserves the right to protect its interests and those of Russian speakers in Ukraine, the Kremlin said.
Meanwhile, Canada has recalled its ambassador to Moscow for consultations.
Canadian Prime Minister Stephen Harper said he was also suspending Canada's preparations for a G8 summit in the Russian resort of Sochi in June.
Ukraine says it has put its army on full combat alert after Russia's parliament approved the deployment of Russian troops.
Acting President Olexander Turchynov said he had also stepped up security at key sites, including nuclear plants.
Mr Obama, the White House said, told Mr Putin that the appropriate way to address any concerns "is peacefully through direct engagement" with the Ukrainian government and international mediating bodies.
"President Obama expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity," the White House said.
Mr Obama told Mr Putin 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", a statement added.
US Secretary of State John Kerry said he had spoken with foreign ministers from Europe and Canada as well as EU foreign policy chief Catherine Ashton and the Japanese envoy to the US "to co-ordinate on next steps".
The UN Security Council held an emergency session on the crisis on Saturday, and NATO has called emergency talks to be held on Sunday.
Russian forces solidify their control
Russian forces have solidified their control of the autonomous region of Crimea, securing key installations and buildings.
There are reports Russia already has about 6000 troops on the ground there.
The unrest in Ukraine has also been spreading with pro-Russian demonstrators clashing with supporters of Ukraine's interim government in several eastern cities as well as on the Crimean peninsula.
Ukraine's acting President Oleksander Turchynov says there is no justification for what he calls Russian aggression against his country.
Earlier, Russia's upper house of parliament approved President Vladimir Putin's request for Russian forces to be used in Crimea.
Russia's Black Sea Fleet is based in Crimea, where many ethnic Russians live.
Kiev has reacted angrily to days of military movements there, accusing Moscow of trying to provoke the new government into an armed conflict and interim president, Olexander Turchynov has called an emergency session of his security chiefs.
The Russian move comes after Crimea's newly-installed pro-Moscow leader, Sergiy Aksyonov, called on Mr Putin to help restore calm.
Mr Putin's request follows days of military activity in Crimea during which unidentified armed men moved in to take over the regional parliament, state television and telecommunications hubs.
Ukrainian Defence Minister Ihor Tenyukh said on Saturday there are now an extra 6000 Russian troops in Crimea, alongside an additional 30 armoured vehicles.
Crimea - a potential flashpoint
Crimea is a centre of pro-Russian sentiment. The region - a peninsula on Ukraine's Black Sea coast - has 2.3 million inhabitants, most of whom identify themselves as ethnic Russians and speak Russian.
The region voted heavily for the recently ousted Ukrainian president Viktor Yanukovych in the 2010 presidential election, and many people there believe he is the victim of a coup - leading to attempts by separatists in Crimea's parliament to push for a vote on whether it should leave Ukraine.
Crimea is legally an autonomous republic within Ukraine, electing its own parliament. The post of Crimean president was abolished in 1995, and it now has a presidential representative and a prime minister appointed by Kiev.