Ten days after Malaysian Airlines Flight 17 was shot from the skies over Ukraine, the situation on the ground in Ukraine has gotten worse.

U.S. officials released satellite images today showing proof that Russian forces have been shelling eastern Ukraine from the Russian side of the border.

The U.S. Office of the Director of National Intelligence, which released the civilian-taken satellite images Sunday, said they show visual evidence that Russia has been firing shells across the border at Ukrainian military forces. Officials also said the images show that Russia-backed separatists have used heavy artillery, provided by Russia, in attacks on Ukrainian forces from inside Ukraine.

Russia has long denied it is actively participating in the shooting war between pro-Moscow separatists and the elected Ukrainian government. Since the downing of the Malaysian jet, the United States has been upping the ante and focusing on evidence showing direct Russian involvement.

Meanwhile, intense ground fighting near the Malaysian plane’s crash scene has halted Dutch investigators in their efforts to gather evidence. The battle just outside the key city of Donetsk is seen by many as a key turning point depending on who is able to command the city.

Government troops were engaged in a pitched battle with rebels on Saturday just outside the separatist bastion of Donetsk and plan to advance next into the city that has been at the heart of the pro-Russian insurgency.

If the army succeeds in retaking Horlivka, a city of almost 300,000 people where fighting was fierce Saturday, they will be within a few miles of Donetsk. Rebels have held sway there since the spring, ruling what they call the Donetsk People’s Republic. Cars created roadblocks out of town Saturday, and the railway station was packed with people desperate to board the next train out.

The military already has ousted rebels from 10 surrounding villages and towns over the past week and blocked roads into and out of Donetsk to prevent supplies from entering the city, according to Andriy Lysenko, a spokesman for the Ukrainian Security and Defense Council.

“The next one will be Donetsk,” Lysenko said, making a bold prediction: “The city will be liberated.”

Ukraine’s President Petro Poroshenko took to the pages of the Washington Post this morning to plead for United States and Western help in halting the Russian-backed separatists violence in his country.

Half a year ago, I was not even considering becoming president of Ukraine. But like a great many Ukrainians at the time, I was disturbed that then-president Viktor Yanukovych constrained Ukraine’s future by rejecting an association agreement with the European Union, choosing a customs union with Moscow instead. Like so many of my countrymen, I believed that for Ukraine to become a modern and successful country, it needed to expand its ties with the West and end widespread corruption and abuses of power. Then, the authorities unleashed a murderous assault on demonstrators in Kiev, and Yanukovych and his partners fled to Russia, leading to Moscow’s decision to annex Crimea and support the violent separatists operating in eastern Ukraine.

Russia’s behavior has only worsened since I took office in June. Over the past several weeks, Ukraine has resisted Russian aggression and continues the fight against the Moscow-backed separatists. Russia has tested us with its transfer of cash, weapons and other equipment to the separatists and its vast anti-Western, anti-Kiev propaganda campaign, but we will not yield to its interference in Ukraine’s sovereignty or to the violence perpetuated by terrorists.

In light of the downing of Malaysia Airlines Flight 17 over Ukraine, Poroshenko is asking the West to significantly step up the pressure on Putin’s Russia.

[T]he West should begin thinking about a larger response to what has happened. As always, the United States should take the lead. Working together with the European Union, Washington can shape a worldwide coalition of nations in support of Ukraine to ensure that these terrorists are not able to strike again.

It is important that the United States has instituted stronger sanctions in response to Russia’s aggression in my country. However, in light of this tragedy, the United States should consider imposing even tougher and wider-reaching sectoral sanctions on Moscow. And Europe needs to follow the lead of the United States and impose sectoral sanctions of its own.

Russia needs to know that the international community is serious. It is long past time for it to act.

Many Putin observers are watching to see how the Russian President reacts if the separatists lose the city of Donetsk. The next few days are critical in how the next stage of the Ukraine crisis develops.