The EU and US have announced travel bans and asset freezes against a number of officials from Russia and Ukraine, BBC reports.
The moves follow Sunday’s referendum in Crimea, in which officials say 97% of voters backed breaking away from Ukraine and joining Russia.
The individuals targeted by the sanctions are seen as having played a key role in the referendum, which Kiev, the US and EU deem illegal.
Pro-Russian forces have been in control of Crimea since late February.
Moscow says the troops are pro-Russian self-defence forces and not under its direct control.
The crisis follows the ousting on 22 February of Ukraine’s pro-Moscow president Viktor Yanukovych, who had sparked months of street protests by rejecting a planned EU trade deal in favour of closer ties with Moscow.
US President Barack Obama said in a press conference that Washington stood “ready to impose further sanctions” depending on whether Russia escalated or de-escalated the situation in Ukraine.
If Moscow continued to intervene in Ukraine, he warned, it would “achieve nothing except to further isolate Russia and diminish its place in the world”.
The EU list of sanctions against 21 officials, which was agreed after a meeting of foreign ministers in Brussels, is expected to be published later on Monday.
“We regret that Russia has so far not engaged in negotiations with Ukraine,” EU foreign policy chief Catherine Ashton said in a press conference after the sanctions were announced.
UK Foreign Secretary William Hague told reporters the list was not “set in stone”.
He said this depends on “how Russia reacts to the referendum in Crimea which has been a mockery of any real democracy, and how they are reacting to the possibility of discussions and direct negotiations with Ukraine over the coming days”.
The US said it had targeted seven top Russian government officials and lawmakers and four Crimea-based separatist leaders with financial sanctions for undermining “democratic processes and institutions in Ukraine”.
The officials include Sergei Aksyonov, the acting leader of Crimea; Dmitry Rogozin, a Russian deputy prime minister; and Valentina Matviyenko, head of the upper house of the Russian parliament. The ousted Ukrainian leader, Viktor Yanukovych, was also on the list.
“Today’s actions send a strong message to the Russian government that there are consequences for their actions that violate the sovereignty and territorial integrity of Ukraine, including their actions supporting the illegal referendum for Crimean separation,” the White House said in a statement.
Monday’s sanctions came hours after Crimea’s parliament declared the region an independent state, following Sunday’s referendum.