More than 14,000 people have been killed in nearly eight years of fighting, and a sharp increase in skirmishes Thursday raised concern that Moscow could use the situation as a pretext for an incursion.
Here is a look at the state of affairs in the rebel-controlled territories in eastern Ukraine:
When Ukraine's Moscow-friendly president was driven from office by mass protests in February 2014, Russia responded by annexing Ukraine's Crimean Peninsula. It then threw its weight behind an insurgency in the mostly Russian-speaking east, known as Donbas.
In April 2014, Russia-backed rebels seized government buildings in the Donetsk and Luhansk regions, proclaimed the creation of ``people's republics'' there and battled Ukrainian troops and volunteer battalions.
The following month, the separatist regions held a popular vote to declare independence and make a bid to become part of Russia. Moscow hasn't accepted the motion, in the hope of using the regions as a tool to keep Ukraine in its orbit and prevent it from joining NATO.
Ukraine and the West accused Russia of backing the rebels with troops and weapons. Moscow denied that, saying any Russians who fought in the east were volunteers.
Amid ferocious battles involving tanks, heavy artillery and warplanes, Malaysia Airlines Flight 17 was shot down over eastern Ukraine on July 17, 2014, killing all 298 people aboard. An international probe concluded that the passenger jet was downed by a Russia-supplied missile from the rebel-controlled territory, but Moscow denied any involvement.
After a massive defeat of Ukrainian troops in the battle of Ilovaisk in August 2014, envoys from Kyiv, the rebels and the Organization for Security and Cooperation in Europe signed a truce in the Belarusian capital of Minsk in September 2014.
The document envisaged an OSCE-observed cease-fire, a pullback of all foreign fighters, an exchange of prisoners and hostages, an amnesty for the rebels and a promise that separatist regions could have a degree of self-rule.
The deal quickly collapsed and large-scale fighting resumed, leading to another major defeat for Ukrainian forces at Debaltseve in January-February of 2015.
France and Germany brokered another peace agreement, which was signed in Minsk in February 2015 by representatives of Ukraine, Russia and the rebels. It envisaged a new cease-fire, a pullback of heavy weapons and a series of moves toward a political settlement. A declaration in support of the deal was signed by the leaders of Russia, Ukraine, France and Germany.
The 2015 peace deal was a major diplomatic coup for the Kremlin, obliging Ukraine to grant special status to the separatist regions, allowing them to create their own police force and have a say in appointing local prosecutors and judges. It also envisaged that Ukraine could only regain control over the roughly 200-kilometer (125-mile) border with Russia in rebel regions after they get self-rule and hold OSCE-monitored local elections _ balloting that would almost certainly keep pro-Moscow rebels in power there.
Many Ukrainians see it as a betrayal of national interests and its implementation has stalled.
The Minsk document helped end full-scale fighting, but the situation has remained tense and regular skirmishes have continued along the tense line of contact.
With the Minsk deal effectively stalled, Moscow's hope to use rebel regions to directly influence Ukraine's politics has failed, but the frozen conflict has drained Kyiv's resources and effectively stymied its goal of joining NATO _ which is enshrined in the Ukrainian constitution.
Moscow also has worked to secure its hold on the rebel regions by handing out more than 720,000 Russian passports to roughly one-fifth of their population of about 3.6 million. It has provided economic and financial assistance to the separatist territories, but the aid has been insufficient to alleviate the massive damage from fighting and shore up the economy. The Donbas region accounted for about 16% of Ukraine's Gross Domestic Product before the conflict.
EFFORTS TO REVIVE PEACE DEAL
Amid soaring tensions over the Russian troop concentration near Ukraine, France and Germany have undertaken renewed efforts to encourage compliance with the 2015 deal, in the hope that it could help defuse the standoff.
Facing calls from Berlin and Paris for its implementation, Ukrainian officials have strengthened criticism of the Minsk deal and warned that it could lead to the country's demise.
Two rounds of talks in Paris and Berlin between presidential envoys from Russia, Ukraine, France and Germany have yielded no progress.
Amid the deadlock in talks, the lower house of Russian parliament this week urged Russian President Vladimir Putin to recognize the independence of Donetsk and Luhansk. Putin signaled, however, that he wasn't inclined to make the move that would effectively shatter the Minsk deal.
ESCALATION OF HOSTILITIES
Ukraine and the rebels accused each other Thursday of intensive shelling along the line of contact in Donetsk and Luhansk.
Separatist authorities claimed that Ukraine mounted a ``large-scale provocation'' and said they returned fire.
Ukraine denied opening fire and said the separatists were shelling government-controlled areas with heavy artillery and mortars. The Ukrainian military command charged that some shells hit a kindergarten in Stanytsia Luhanska, wounding two civilians, and cut power supply to half of the town.
Yasar Halit Cevik, head of the OSCE monitoring mission, said it reported 500 explosions along the contact line between Wednesday evening and 11:20 am Thursday. Cevik told the United Nations Security Council that the tension appeared to be easing after that with about 30 explosions reported, adding ``it is critically important to de-escalate immediately.''