U.S. President Barack Obama says the United States and Germany are united in their determination to impose "costs" on Russia for its actions in Ukraine, accusing Russia of providing "significant support" to "heavily armed militants" in eastern Ukraine.
He said those costs include "coordinated sanctions" that will increase Russia's diplomatic and economic isolation.
Speaking at the White House with visiting German Chancellor Angela Merkel on May 2, Obama also said that Germany and the United States had an "unwavering Article 5 commitment to the security" of NATO allies.
Article 5 is a clause in the military alliance's treaty that requires all NATO members to come to the aid of any NATO state that comes under foreign attack.
Obama said part of that Article 5 commitment was the use of German military aircraft that are joining with NATO air patrols over the Baltics, which border Russia.
Obama's remarks come after Kyiv on May 2 launched military operations at the eastern Ukrainian city of Slovyansk
-- where pro-Russian separatist forces have seized government buildings, blockaded the city, and detained a group of international monitors from the Organization for Security and Cooperation in Europe (OSCE) that includes four Germans.
Obama said that "as Ukrainian forces move to restore order in eastern Ukraine, it is obvious to the world that these Russian-backed groups are not peaceful protesters."
He called eastern Ukraine's pro-Russian separatists "heavily armed militants who are receiving significant support from Russia."
Obama said that Ukraine's government in Kyiv "has the right and responsibility to uphold law and order within its territory" and that "Russia needs to use its influence over these paramilitary groups so they disarm and stop provoking violence."
Obama also said he and Merkel were "united in our outrage over the appalling treatment" of the OSCE monitors who have been seized by pro-Russian separatist forces in eastern Ukraine.
Merkel said if Russia "continues to destabilize eastern Ukraine and disrupt" Ukraine's May 25 elections, further sanctions will be imposed against Russia's leadership.
The United States and the European Union have already targeted dozens of Russian individuals and pro-Russian Ukrainians with sanctions over Russia's annexation of the Crimean Peninsula.
Further sanctions targeting Russia's energy sector have been discussed by officials in Washington and Brussels.
Merkel noted that six EU countries were 100 percent dependent on Russian natural-gas supplies.
But she said the EU was looking at a long-term strategy to decrease Europe's dependency on Russian energy.