As Ukrainian government forces close in on pro-Russian rebels in the country's eastern region, a row between Moscow and the West on ending the conflict threatened to come to a boil.

U.S. President Barack Obama spoke to his Russian counterpart, Vladimir Putin, by phone on Friday, and had a meeting with reporters later where he expressed his frustration with Moscow.

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Obama said the U.S. had done "everything we can do" but that "sometimes people don't always act rationally, and they don't always act based on their medium- or long-term interests."

British Prime Minister David Cameron wrote to the other leaders of NATO urging the military alliance to plan measures aimed at protecting their allies against any aggression by Russia.

"Six months into the Russian-Ukraine crisis we must agree on long-term measures to strengthen our ability to respond quickly to any threat, to reassure those allies who fear for their own country's security and to deter any Russian aggression," Cameron said in his letter.

The diplomatic impasse comes in the wake of new sanctions that the U.S. and the European Union imposed on Russia.

The West has accused Moscow of not doing enough to help end the conflict in eastern Ukraine where government forces are fighting pro-Russian separatists.

Government troops recaptured two cities, Krasnogorovka and Staromikhailokva, from rebel forces on Saturday, bringing them closer to Donetsk, which is the separatists' main stronghold.

Russia maintains it does not provide military support to the rebels and accuses the West of enforcing a containment policy on Russia.

The Russian Foreign Ministry has also said the EU is using "double standards" by imposing sanctions on Russia's defense industry while allowing arms sales to Ukraine.

The West and Moscow have also traded accusations on the Malaysia Airlines flight MH17 crash over eastern Ukraine on July 17.

The U.S. suspects that separatists hit the plane with a Russian-supplied missile, mistaking it for military aircraft.

Moscow and the rebels deny responsibility for the crash, which killed nearly 300 people, and blame it on the government of Ukraine.

TagsRussia vs. United States, russia vs. ukraine, Russia vs. European Union, Barack Obama, david cameron, Ukraine fighting