Russia has joined the 56 other members of the Organization for Security and Co-operation in Europe in a consensus decision to send a six-month monitoring mission of the rights and security group to Ukraine, diplomats said.
The decision came after several failed attempts in recent weeks to agree on such an observer mission to help defuse the tense situation in the former Soviet republic.
Western diplomats have blamed Russia for the delay in agreeing the mission.
The Kiev-based mission will initially consist of 100 civilian monitors but that number may later expand by another 400 personnel.
It will initially be deployed in nine places,including Donetsk, a major city in largely Russian-speaking eastern Ukraine.
The text of the decision does not mention Crimea, which Russia annexed from Ukraine this week.
Earlier today Russian President Vladimir Putin signed legislation that completed the process of absorbing Crimea into Russia.
In a Kremlin ceremony shown live on state television, Mr Putin signed a law on ratification of a treaty making Crimea part of Russia
He also signed legislation creating two new Russian administrative districts: Crimea and the port city of Sevastopol.
A spokesman for the president said Russia would respond in kind to US sanctions announced yesterday.
"We will respond every time," Kremlin spokesman Dmitry Peskov told reporters.
"We have responded to the first sanctions. Now we will also react to these, of course. They will not remain without a response."
Visa and MasterCard have stopped servicing the credit cards of customers using Russian banks affected by the US sanctions.
Bank Rossiya, used by close associates of Mr Putin, said in a statement that Visa and MasterCard had "without warning stopped providing its payment services to the bank's clients."
An affiliate bank, Sobinbank, which is 100%-controlled by Bank Rossiya, was also affected.
Bank Rossiya was hit by a fresh wave of Washington sanctions announced yesterday, which also targeted several billionaire businessmen believed to be close to Mr Putin.
Clients of the banks are now unable to use their credit cards to make purchases or to withdraw money from cash machines not belonging to the banks.
SMP Bank, which is controlled by brothers Arkady and Boris Rotenberg who were among those blacklisted, also said its clients were unable to make Visa and MasterCard payments.
US President Barack Obama said the list of Russians targeted by sanctions had been extended to include oligarchs and bankers believed to be close to Mr Putin, including Gennady Timchenko and Yuri Kovalchuk.
Bank Rossiya was listed separately as a target for sanctions.
Americans and American businesses, such as Visa and MasterCard, are forbidden from any dealings with anyone targeted by the sanctions and any assets they might have in the US are frozen.
Russia sees no impact from sanctions
Russia's Deputy Finance Minister has said he sees no severe consequences so far for Russia's financial sector from sanctions that have been imposed by the US and the EU.
Alexei Vladimirovich Moiseev's comments come after the Fitch ratings agency revised its outlook for Russia to negative from stable.
Europe has added 12 people to a list of those subject to travel bans and asset freezes for their part in Russia's seizure of Crimea.
French President Francois Hollande declined to give details of the names added to the list, which was agreed among the EU's 28 leaders after six hours of talks yesterday.
However, he said it included Russians and Crimeans and raised the total number of people subject to EU sanctions to 33.
German Chancellor Angela Merkel said preparations would also be made for possible trade and economic measures against Russia, if it moves beyond Crimea into southern and eastern Ukraine.
Ms Merkel confirmed that the EU was calling off an EU-Russia summit scheduled for June, severing political ties.
"We are ready to start stage three if there is further escalation with a view to Ukraine, those are economic sanctions and we asked the European Commission today to do preparatory work for possible economic sanctions," Ms Merkel said.
One of the major difficulties the EU faces in matching the US response is maintaining unity, since EU sanctions must be agreed unanimously among all member states.
With several countries depending on Russia for oil and gas or having close trade, investment and business links to Moscow, there was a reluctance to push too hard, too quickly.
Instead, the EU is looking to steadily increase the pressure.
The Irish export trade to Russia has more than doubled in the past five years, with over €630m in goods being exported there.
Speaking on RTÉ's Morning Ireland John Whelan, a consultant in export trade, said he is concerned that sanctions against Russia will badly affect Irish exporters.
UN chief urges talks between Ukraine and Russia
UN Secretary General Ban Ki-moon has urged Ukraine and Russia to meet for talks to prevent the crisis between them becoming "uncontrollable".
"What is important at this time is for Ukraine and Russian authorities to sit down together and engage in direct and constructive dialogue," said Mr Ban.
He was speaking after meeting the acting Ukrainian President Oleksandr Turchynov in Kiev.
Mr Turchinov said Ukraine would never accept Russia's annexation of Crimea.