Pro-Russian rebels have launched a new push to take control of more of eastern Ukraine from government forces and if a renewed ceasefire cannot be agreed, there are fears that the conflict could spiral out of control.
Ukraine’s Western allies accuse Russia of sending in troops and armour to help the rebels – an allegation repeatedly denied by the government in Moscow.
More than 5,000 people have lost their lives in a crisis that few saw coming.
Why has eastern Ukraine returned to full-scale conflict?
Fighting started in April 2014 and raged for months until Ukraine and the separatists came to a deal on 5 September to halt the violence and free prisoners.
But the ceasefire never held entirely. Both sides used the relative lull to build up their forces and for months the rebels tried to seize Donetsk airport, a strategic and symbolic asset, from government forces.
With the start of 2015, a new rebel push began and by 22 January the airport was in their hands.
Donetsk airport – coveted prizeViolence has flared the length of the September ceasefire line and the casualties have mounted:
Civilians were killed by shells and rockets in rebel-held areas, particularly the cities of Donetsk and Luhansk
Thirty people died in the Ukrainian port city of Mariupol, in an artillery attack that international observers said came from rebel-held areas
The rebels seized Vuhlehirsk, a town west of the major road and rail hub of Debaltseve
The separatists are now besieging Debaltseve, which would give them control of a tongue-shaped wedge of territory that divides part of the Donetsk and Luhansk regions, which they see as their own.
Ghost town of Vuhlehirsk
Is there any hope of a new ceasefire?
The best hope is a renewed deal in Minsk on 11 February, with planned talks between the leaders of Germany, France, Ukraine and Russia. The terms of the deal appear to be a revision of the failed 5 September ceasefire.
A previous attempt to renew the ceasefire failed on 31 September because some of the rebel negotiators did not turn up and those who did were not prepared to discuss a truce, international observers said.
Russia’s President Vladimir Putin then put forward a proposal widely seen as unacceptable to Ukraine and the West. A new plan was proposed by the leaders of France and Germany on visits to Kiev and Moscow.
What is in the new ceasefire plan?
A broad demilitarised zone 50-70km (31-43 miles) wide, straddling the front line, and wider autonomy for the Russian-backed separatists are the main points of the plan, according to French President Francois Hollande.
Details have not been made public and sticking points will include: the status of the rebel-held territories, who controls the border with Russia, and the withdrawal of heavy weapons.
Russia is thought to want Ukraine to pull its weapons back, but there are fears that the rebels, with backing from Moscow, could at some stage seize on such a withdrawal to grab more territory.
If no deal is reached, both sides have already announced plans to bolster their forces by tens of thousands. With spring on its way, the fear is that the conflict could escalate still further.
Ukraine’s war: The human cost
5,358 people killed and 12,235 wounded in eastern Ukraine
Fatalities include 298 people on board flight MH17 shot down on 17 July
224 civilians killed in three-week period leading up to 1 February
5.2 million people estimated to be living in conflict areas
921,640 internally displaced people within Ukraine, including 136,216 children
600 000 fled to neighbouring countries of whom more than 400,000 have gone to Russia
Source: Figures from UN High Commissioner for Human Rights 3 February, and UN report, 21 January
Why did the September ceasefire break down?
Each side accused the other of tearing up the peace deal.
For the government, it was the rebels’ decision to hold their own local elections in November 2014, in defiance of the Kiev authorities.
The separatists were then angered by the government decision to scrap the special status of their two regions, Donetsk and Luhansk.
Among the terms of the truce were a commitment to pull back heavy guns from the front line by at least 15km (9 miles), a release of prisoners and an agreement for international observers to monitor the truce and a buffer zone on the Ukrainian border with Russia.
Ukraine would also grant wider self-rule to Donetsk and Luhansk.
Both sides used the ceasefire to rearm, but the rebels now appear to have better quality tanks and weapons than the government.
Who has the upper hand now?
The rebels certainly have made big gains, with the capture of Donetsk airport and advances around Debaltseve.
The airport gave them a strategic asset a few miles from the centre of Donetsk city, their biggest stronghold.
Even before the airport had been captured, Ukraine accused separatist forces of seizing more than 500 sq km (194 sq miles), mainly around Debaltseve and Mariupol.
Seizing Debaltseve would give the rebels far greater control of Donetsk and Luhansk.
But casualties on both sides have been heavy and journalists say the separatists have suffered major losses.
Ukrainian forces made significant gains last summer, many of which have not been reversed.
But the separatists opened up a front on the coast of the Sea of Azov before the September ceasefire and are now in range of the port city of Mariupol.
Why did the fighting start in the first place?
In April 2014, pro-Russian activists seized control of government buildings in towns and cities across the Donetsk and Luhansk regions.
The events were a repeat of what had happened in Ukraine’s Black Sea peninsula of Crimea.
Gunmen there seized government buildings in February 2014 and raised the Russian flag a week after elected President Viktor Yanukovych, friendly to Moscow, fled during massive pro-EU street protests in Kiev.
An early attempt by nationalists to remove the status of Russian as a second language alarmed many Russian speakers, and officials in Moscow portrayed the new leaders in Kiev as US-backed Ukrainian nationalists bent on violating minority rights.
A flawed referendum on joining Russia was quickly held in Crimea and within a month the peninsula’s annexation was complete.
There was little bloodshed in Crimea, but Ukraine’s fledgling revolutionary government was in no position to fight back, with only 6,000 troops reportedly ready for combat.
However, when pro-Russian separatists made a move on Ukraine’s industrial east and Russian forces appeared to be building up on the borders, the authorities in Kiev ordered an “anti-terrorist operation”.
What does Russia want?
Russia is part of international attempts to restore a ceasefire to eastern Ukraine and also agreed in October to turn its gas supply to Ukraine back on.
But President Vladimir Putin’s ultimate aims are unclear. Russia has not officially recognised Ukraine’s two rebel regions but Mr Putin recently referred to them as people’s republics for the first time.
He has also revived the idea of “Novorossiya” – a big swathe of southern and eastern Ukraine that used to be part of the Russian empire.
The real question is the extent to which Russia is prepared to back the rebels’ quest for territorial gain.
The Kremlin has repeatedly denied supplying troops and sophisticated military hardware to the rebels.
Alexander Zakharchenko, the Donetsk rebel leader, said last summer that 3-4,000 Russian citizens had been fighting alongside the rebels, although Russia insists they are not fighting in an official capacity.
When Ukrainian President Petro Poroshenko last month estimated that as many as 9,000 Russian troops were backing the rebels, Russia said he had no evidence.
And if Russia is backing the rebel offensive, would it support a push towards Mariupol?
It has awarded a contract for a $3bn (£2bn) bridge from the Russian mainland to Crimea, but if the rebels captured Mariupol, then that could pave the way for a land corridor to the peninsula.