Kiev - The crisis in Ukraine began in November last year as a protest against the government dropping plans for closer economic ties with Europe.
But it has since snowballed into the biggest stand-off between the West and Russia after Moscow annexed Ukraine's Crimean peninsula and the port city of Sevastopol in February, following the ouster of Ukrainian president Viktor Yanukovich.
Russia's refusal to remove its troops from Crimea or renounce its support for pro-Russian separatists in Ukraine have presented a serious challenge to global principles about sovereignty.
Moscow further raised tension in eastern Europe by massing tens of thousands of troops in the regions bordering eastern Ukraine.
Fighting in various parts of Ukraine has continued unabated since April between government forces and the separatists who want a union with Russia. The rebels have seized government buildings and proclaimed at least two "people's republics" in separate provinces.
The new Ukrainian government under President Petro Poroshenko, who was elected in May, has vowed a hard line against the separatist insurgency, resulting in more intense fighting in eastern areas like Donetsk, a rebel stronghold.
Nato secretary-general Anders Fogh Rasmussen has called the crisis between Ukraine and Russia the greatest threat to European security since the end of the Cold War.
The United States has slapped sanctions on Russia to punish it for its aggression in Ukraine, and sent military assets and troops to the region in a show of force.
But US President Barack Obama has ruled out military action in Ukraine, with Moscow showing little interest in compromising.
Some commentators, however, say the downing of Flight MH17 over eastern Ukraine could prove a turning point for the political stalemate by prompting global leaders to take tougher action to end the fighting.
The Malaysia Airlines jetliner was brought down near the village of Grabove, about 40km from the border with Russia and near Donetsk, which has seen fierce fighting in recent months.
"This outrageous event underscores that it is time for peace and security to be restored in Ukraine," said Mr Obama, adding that Russia had to do more to quell the rebel violence.
He and several world leaders, including German Chancellor Angela Merkel, have called for an immediate ceasefire so investigators could reach the crash site.
"Moscow may have a last chance now to show that it really is seriously interested in a solution," Germany Foreign Minister Frank-Walter Steinmeier told Bild am Sonntag newspaper.
"Now is the moment for everyone to stop and think to themselves what might happen if we don't stop the escalation."
Stop and think "Moscow may have a last chance now to show that it really is seriously interested in a solution. Now is the moment for everyone to stop and think to themselves what might happen if we don't stop the escalation." - German Foreign Minister Frank-Walter Steinmeier