NATO estimates that 1,000 Russian soldiers are in Ukraine, aiding pro-Russian separatists in the country's east.
In a blunt statement Aug. 29, NATO said Russian troops have "illegally crossed the border" as part of a "dangerous pattern over many months" to destabilize its neighbor.
What does the United States say?
Samantha Power, U.S. ambassador to the United Nations, told an emergency meeting of the U.N. Security Council on Aug. 28 that Russia has "outright lied" over its military activity inside Ukraine.
Later that day, President Obama said the United States "is not taking military action to solve the Ukrainian problem" but trying to mobilize international pressure on Moscow.
What does Russia say?
Russia has repeatedly denied claims it is supporting rebels in Ukraine's east with weapons and expertise. It continues to deny that its troops are operating in Ukraine, despite NATO's release of satellite images showing Russian tanks, armored personnel carriers and artillery moving around eastern Ukraine.
What are international leaders doing to pressure Russia to back off?
The European Union gave Russia a one-week ultimatum to scale back its intervention in Ukraine or face additional sanctions. The EU imposed fresh sanctions on the country in late July that aimed to further cripple the nation's economy. Although previous sanctions focused on specific businesses and individuals, the ones set in July were designed to hit major pillars of the Russian economy, including oil and gas supplies and technology, banking and finance, and arms sales. Close associates of Putin were also targeted.
Moscow imposed a ban on food imports in retaliation for the sanctions. By mid-August, suppliers and consumers are facing shortages and price hikes on staples such as fish and fruit, as well as gourmet items such as Italian Parmesan and French Brie cheese.
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What does Putin hope to gain by meddling in Ukraine?
That's the million dollar question. Russia and Ukraine have a long history of cultural, political and economic connectedness. The Rus people who founded the Russian state with a capital in Moscow centuries ago got their start in Ukraine, which they settled first. Russia views Ukraine as a strategic buffer against the NATO states of Western Europe. Ukraine has Europe's third-largest population and enormous agricultural and industrial potential. Vladimir Putin wants it for his Customs Union, an alternative economic alliance to the European Union.
Ukraine's decision to associate with the EU instead will require addressing political and economic corruption in Ukraine. Putin says he's coming to the aid of ethnic Russians whom he claims are at risk of mistreatment by an independent and nationalist Ukraine. However, many analysts say that the cultural connection between the countries means such reforms in Ukraine could give rise to demands for similar reforms in Russia, which would threaten Putin's corrupt regime.
Why is information about Russian troops in Ukraine vague?
Russia's military adventure in Ukraine has relied on a combination of ethnic Russian-Ukrainian insurgents that it has armed and trained, non-uniformed Russians who've joined the fighting, and Russian special forces and military intelligence personnel who wore no insignias on their uniforms. That approach has made it easier for Russia to deny direct involvement and claim that the fighting in east Ukraine was a legitimate insurgency rather than the precursor to a Russian invasion.
What is the death toll from the conflict?
Nearly 2,600 people have died in clashes between Ukrainian armed forces and Russian-backed rebels since April, according to a U.N. report.
How did this all start?
The crisis began last November when thousands of protesters took to the streets of Kiev after pro-Russian Prime Minister Viktor Yanukovych, who was later ousted and fled the country, refused to sign an agreement that would have integrated Ukraine's economy closer to the European Union, making it less dependent on Russia.
In February, Russia seized the Crimean Peninsula from Ukraine without any resistance. In March, Crimea voted in favor of a referendum on joining Russia — one the West considered to be illegal and rigged in Russia's favor.