Relations between the United States and Russia can be described as complicated, said a senior Obama administration official who is currently on a visit to New Delhi.
Wendy Sherman, U.S. Under Secretary of State for Political Affairs, said that though Washington and Moscow disagreed on a lot of issues, there was agreement for instance on bringing Iran to the negotiating table through sanctions to convince Tehran to accept restrictive use of its nuclear capabilities. Similarly, she said, both the United States and Russia had worked on an agreement to deal with the civil war in Syria.
However, she said that what was not acceptable to Washington was Russia's move on the Central Asian Republic of Ukraine.
"We were and are very concerned about the illegality of the taking over of the Crimea by the Russians," Sherman said.
She was referring to recent news of Russia sending arms and drone aircraft into eastern Ukraine and once again concentrating its troops along the tense border.
According to a report in the LA Times web site, Russia, which is supporting separatists battling the central Ukraine government in Kiev, has built up its air defense systems to the highest levels since August.
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The web site quoted Marie Harf, a State Department spokeswoman, as saying in a statement recently that Russia now has deployed more troops and military material on its border with Ukraine than at any time since October, when fighting in eastern Ukraine was heavy.
The Russians and separatists are conducting increasingly complex training missions that leave "no doubt" that Russian troops are present in eastern Ukraine, despite Moscow's denials, Harf said in her statement.
The training operations involve Russian drone aircraft that are "unmistakable signs of Russia's presence," Harf said.
The Russians and their separatist allies "continue to violate the terms" of a cease-fire agreement signed February 15 in Minsk, Belarus, Harf said.
The cease-fire sought to end months of fighting between the Kiev government and the separatists. But it has been violated repeatedly, and Western officials are worried that Moscow is planning to escalate the conflict again.
According to a report by ft.com, Russia has accused the United States of deploying military trainers in the combat zone in east Ukraine, while Washington has charged Moscow with organizing a military build-up in the war-torn region.
The United States warned that the military manoeuvres could cast a shadow over hopes of a European-brokered truce in Minsk that was supposed to hold through the summer.
Kiev and Western countries, according to ft.com, remain concerned that Russian-backed forces could launch a new assault to cut off Ukrainian exports through Mariupol and as part of attempts to create a land bridge linking Russia to Crimea.