Russia's State Duma has ratified an agreement that makes Crimea part of the Russian Federation. The agreement is expected to be endorsed by the upper house of parliament on March 21.
The annexation of Crimea -- an autonomous republic of Ukraine -- has been rejected by the United States and other Western nations as illegal.
In a resolution on March 20, Ukraine's parliament said Kyiv will never recognize Russia's move.
The resolution said Ukraine "will never cease to fight for the liberation of Crimea as long and painful as this can be."
Russian President Vladimir Putin and Crimea's secessionist leaders signed the treaty on March 18 following a controversial referendum in the majority ethnic Russian region, which overwhelmingly backed the move.
Russia sent thousands of troops to Crimea, the base of its Black Sea Fleet, late last month after the ouster of Moscow-backed President Viktor Yanukovych, citing concern for ethnic Russians.
On March 19, Moscow began issuing passports to residents of Crimea saying they are now Russian citizens.
Meanwhile, Ukraine's government announced plans to withdraw its soldiers from Crimea after two Ukrainian naval bases, including the Navy headquarters in Sevastopol, were stormed by Russian troops and pro-Russian forces.
The Ukrainian Navy commander was detained when the Navy headquarters in Sevastopol was stormed, but he was released on March 20.
Meanwhile, UN Secretary-General Ban Ki-moon is holding talks in Moscow before flying to Kyiv on March 21 in an effort to reduce international tensions.
Ban met with Russian Foreign Minister Sergei Lavrov and was expected to meet President Vladimir Putin later on March 20.
His visit to Moscow comes after the U.S. and the Russian envoys clashed at a UN Security Council meeting the previous day over the Ukraine crisis.
U.S. Ambassador Samantha Power compared Russia's takeover of Crimea to theft.
"Now the referendum [in Crimea] has taken place but the national and international legal status of Crimea has not changed," she said. "A thief can steal property but that does not confer the right of ownership on the thief. In closing, Madam President [of the Security Council], let me just emphasize again -- what Russia has done is wrong as a matter of law; wrong as a matter of history; wrong as a matter of policy; and dangerous."
Russia's envoy Vitaly Churkin warned that the U.S. ambassador's comment could jeopardize Russia's willingness to cooperate with Washington on other issues.
"Mrs. Power began [her statement] with references to [Russian writers Lev] Tolstoy and [Anton] Chekhov but ended up descending to the level of tabloid media," he said. "We categorically reject offensive statements towards our country. If the U.S. delegation [to the UN] counts on our cooperation at the [UN] Security Council on other issues, then Mrs. Power ought to grasp it really well."
On March 20, Ukraine's ambassador to the United States in Geneva, Yuriy Klimenko, said Russia may be preparing a further military incursion into Ukrainian territory.
European Union leaders begin a two-day summit in Brussels on the same day to agree on a further response to Russia's annexation of Crimea.
The EU has already imposed visa bans and asset freezes on eight Crimeans and 13 Russian officials over Russia's activities in Crimea.
With reporting by AFP, Reuters, and AP