http://www.bbc.com/news/world-europe-28969784
By Laurence PeterBBC News
The conflict in eastern Ukraine has intensified, with the Kiev government and its Western allies accusing Russia of sending in troops and armour to help the rebels.
A new front has opened up on the coast, with the pro-Russian separatists now controlling Novoazovsk, a small town on the way to the strategic port city of Mariupol.
Heavy fighting continues around the rebel-held cities of Donetsk and Luhansk.
Why did the fighting erupt in eastern Ukraine?
In April, pro-Russian activists stormed government buildings across the Donetsk and Luhansk regions. Many of them were armed, and a tense stand-off with the Kiev authorities escalated.
Russian politicians and media portrayed the Kiev leaders as Ukrainian nationalists bent on violating Russian-speakers' rights.
Russia's annexation of Crimea, a Black Sea peninsula, in March gave momentum to the rebels' drive to break away from Kiev.
The activists were enraged by the installation of a new pro-Western government in Kiev after massive street protests had toppled President Viktor Yanukovych in February.
Mr Yanukovych, friendly to Moscow, had been elected democratically. But there was widespread discontent over economic stagnation, corruption and cronyism, and his refusal to sign up to a partnership with the EU.
He used to be a powerful politician in the east, where the majority Russian-speakers mostly feel a strong bond with Russia.
Is the Russian military helping the rebels?
Yes. The question is, how much of this help is orchestrated at a senior level in Russia.
The Kremlin has repeatedly denied allegations by Kiev and the West that it is supplying troops and sophisticated military hardware to the rebels.
But the rebel leader in Donetsk, Alexander Zakharchenko, says 3-4,000 Russian citizens have been fighting alongside the rebels. He said Russian soldiers were continuing to join the rebel ranks, rather than "going to the beach".
Nato has released satellite pictures showing, it says, Russian artillery and military columns inside Ukraine. And Nato officials say at least 1,000 Russian troops are operating in Ukraine.
Is Ukraine getting help from the West?
The US says it is supplying Ukraine with non-lethal military equipment,including radios, vehicles and "non-lethal individual tactical gear".
The US is also sharing some intelligence information with Ukraine, the New York Times reports. But that does not include real-time data on potential targets, the paper's sources say.
Ukraine is not in Nato - so that restricts the kind of military help the West can provide legally.
There are unconfirmed reports that US military advisers have been helping Ukraine in its major offensive against the rebels in July-August.
The EU has put in place a wide-ranging economic support package for Ukraine, to help Kiev manage its debts and balance-of-payments problems.
Are we seeing a repetition of what happened in Crimea?
There are clear parallels, but also some important differences.
There are many reports of well-trained Russian-speaking soldiers operating in eastern Ukraine without insignia on their uniforms or vehicles.
Similar soldiers, nicknamed "little green men", seized key installations in Crimea. Only later did Russia admit that they came from Russia. Earlier Russia had claimed that all the armed men were local separatists.
The separatists in Donetsk and Luhansk declared independence after referendums on 11 May, which were not recognised by Kiev or its Western allies.
A similar controversial referendum was held in Crimea, before the region was incorporated into Russia - an act condemned internationally.
But Russia has long had a sovereign naval base in Crimea - at Sevastopol - and native Russian-speakers are by far the largest group on the peninsula. Crimea was transferred to Ukraine from Russia in 1954, in Soviet times.
In eastern Ukraine, the population is more mixed ethnically and politically - and it is a far bigger area to control than Crimea.
Why does this matter for the rest of Europe?
It is widely recognised to be Europe's biggest security crisis since the Cold War period, which ended in 1989.
For years the EU has been developing closer ties with Ukraine, yet Russia has been urging Ukraine to join its customs union with some other ex-Soviet countries.
Ukraine is on a geopolitical fault line - torn between east and west, its internal tensions were not resolved after the USSR's collapse in 1991.
Nato and the EU accuse Russia of bullying Ukraine and say such tactics are unacceptable.
Most of the ex-communist countries in eastern Europe, once dominated by Moscow, demand a tough stance towards Russia's President Vladimir Putin.
The EU and US have ratcheted up sanctions against Russia, hoping to force Mr Putin to back down over Ukraine and enter serious negotiations with Kiev.
War in eastern Ukraine: The human cost
- At least 2,119 people had been killed and 5,043 wounded since mid-April, a UN report on 7 August said
- 951 civilians have been killed in Donetsk region alone, the official regional authorities said on 20 August
- Official casualty counts only record certified deaths while in some particularly dangerous parts of the war zone, such as Luhansk region, victims are said to have been buried informally, for instance in gardens
- Rebels (and some military sources) accuse the government of concealing the true numbers of soldiers killed
- 155,800 people have fled elsewhere in Ukraine while at least 188,000 have gone to Russia
