"North Atlantic Council will meet tomorrow followed by NATO-Ukraine Commission," NATO's Secretary General Anders Fogh Rasmussen said late Saturday on Twitter.
The council consists of the permanent representatives, or NATO ambassadors, of member countries.
Ukraine's Foreign Minister Sergei Deshchiritsya asked NATO to look at possible ways to help it protect its territorial integrity, he told reporters.
Russia's upper house of parliament Saturday approved a request from President Vladimir Putin to use armed forces inside Ukraine "until the social and political situation in that country is normalized."
Ukraine responded by placing its military on high alert.
Putin said the measure was justified because of a threat to Russian citizens, ethnic Russians and military personnel in the Crimean peninsula.
It was met with strong condemnation from the United States, and Britain, which were reportedly considering backing out of preparatory talks for June's G8 summit of leading nations in Sochi.
US President Barack Obama held a 90-minute phone call with Putin late Saturday, and called on Russia to withdraw its forces back to bases in Crimea, where several thousand Russian troops are stationed.
Canada, whose inhabitants include more than 1.2 million people who consider themselves ethnically Ukrainian, withdrew its ambassador to Moscow Saturday and said it would not participate in pre-G8 talks.
Britain's Foreign Minister William Hague was due to visit Kiev on Sunday, he said by Twitter on Friday.
Ukrainian former boxing champion and opposition leader Vitaly Klitschkko called for bloodshed to be avoided in finding a solution to the conflict with Russia.
"We should not solve this question with the help of violence and weapons, but with a path to consultations," he said in a statement by his party Udar (punch).
He said the heightened tensions on the Crimean peninsula threatened the country's integrity, and condemned Russia's threat of military intervention as meddling in the internal affairs of the former Soviet republic.
"We must end all speculation of separatism," said Klitschko, who has said he wants to run in presidential elections on May 25.