The Organization For Security And Cooperation In Europe (OSCE) has agreed that up to 500 OSCE civilian observers will be deployed to Ukraine for six months.
OSCE member states, including Russia, agreed to the move at a meeting late on March 21 in Vienna.
Russia had blocked earlier efforts by the OSCE to deploy observers.
The mandate does not permit the observers to deploy to Crimea, which Russia annexed this week in the face of opposition from Western states, who say they still consider Crimea part of Ukraine.
Andrei Kelin, Russia's chief OSCE envoy, said the peninsula was off limits for the observers because "Crimea is a part of the Russian Federation."
But U.S. chief envoy Daniel Baer told reporters that because "Crimea is Ukraine ... they should have access to Crimea."
In Berlin, German Foreign Minister Frank-Walter Steinmeier said that — although the decision on the monitors is "not the end of the crisis ... it is a step that helps support our efforts toward de-escalation."
The mandate says the monitors will initially deploy to Kherson, Odesa, Lviv, Ivano-Frankivsk, Kharkiv, Donetsk, Dnepropetrovsk, Chernivtsi, and Luhansk, with a head office in Kyiv.
The mandate says the mission will consist initially of 100 monitors, but could be expanded to 500 depending on developments.
Earlier, U.S. national security adviser Susan Rice said the Obama administration was watching Russian troop movements near the Russia-Ukraine border.
Rice noted that Moscow says the troops are conducting military exercises, but she said U.S. officials were watching with skepticism because Moscow's words haven't always matched Russian actions.
Next week's trip to Europe by President Barack Obama will demonstrate that Russia is facing increasing international isolation over its annexation of Ukraine's Crimea, she added.
Rice said Obama will show that "the United States is leading the international community in supporting the government of Ukraine and the people of Ukraine" and imposing "costs" on Russia over its Ukraine intervention.
The United States and European Union say the Crimea annexation was illegal and have imposed sanctions on Russian officials.
These developments come after Moscow completed the process of making Ukraine's Crimea part of Russia, as Ukraine's prime minister, in a highly symbolic move, signed a political cooperation agreement with the European Union.
And the European Union released the names of 12 more Russians and Ukrainians who are being hit with sanctions to punish Russia over the Crimea annexation.
Both the United States and the European Union expanded their respective sanctions lists. The United States imposed sanctions on allies of President Vladimir Putin, as well as Bank Rossiya, which has close ties to the Russian leadership.
In retaliation, the Russian Foreign Ministry banned nine U.S. lawmakers and officials from traveling to Russia.
With its 12 additions, the EU has now imposed sanctions on more than 30 Russians and Ukrainians in the past week.
The new names on the list include Putin advisers Sergei Glazyev and Vladislav Surkov, Federation Council Chairwoman Valentina Matviyenko, and Sergei Naryshkin, the State Duma lower house.
Dmitry Kiselyov, the TV journalist who warned the United States could be turned "into radioactive ash" is also on the list, as well as Igor Turchenyuk, who the EU says is "the de facto commander of Russian troops deployed on the ground in Crimea.
Meanwhile, Russian Deputy Prime Minister Dmitry Rogozin and U.S. Senator John McCain (Republican-Arizona) have been trading jibes on Twitter over the sanctions that have been imposed against them by each other’s government over the Crimea crisis.
McCain, who was among nine U.S. officials hit with a Russian travel ban on March 20, mocked the sanctions, writing (@SenJohnMcCain), "I guess this means my spring break in Siberia is off, Gazprom stock is lost & secret bank account in Moscow is frozen."
Rogozin (@Rogozin) responded, "Well, Johnny, never say never!"
McCain, who has visited Ukraine twice in recent months, is regarded as a staunch opponent of the Kremlin. He also tweeted that his support would never cease for "the freedom, independence, and territorial integrity of Ukraine."
Both Washington and the European Union have imposed sanctions against Rogozin.
With reporting by dpa, AFP, and Reuters