A warplane has attacked a rebel-held town with rockets in eastern Ukraine, shattering homes and killing 11 people.
Rockets struck the town of Snizhne in Donetsk region around 07:00 (04:00 GMT), hitting a block of flats and a tax office.
The rebels blamed the attack on Ukraine's air force - a claim denied by Ukrainian sources.
Security forces have been pushing the rebels back to the city of Donetsk.
Fighting has also raged in the neighbouring region of Luhansk, with rockets hitting a southern suburb of Luhansk city on Monday.
Nato says it has observed a significant increase of Russian troops along the Ukrainian border, bringing their number to up to 12,000. Russia had an estimated 40,000 troops in the area two months ago before pulling them back.
Russia, which annexed Crimea from Ukraine after a disputed referendum in March, denies arming and facilitating the pro-Russian rebels.
Ukrainian officials have said that only Russia could have shot down a transport plane near the border on Monday but a Western defence expert cast doubt on the claim.
On Wednesday, EU leaders will discuss the possibility of further sanctions against Russia at their summit in Brussels.
Ukrainian officials said 11 people had been killed and eight injured, including a child. Earlier, they had put the toll at four while rebels spoke of around 10 civilians being killed.
A video posted on YouTube on Tuesday shows men in what appears to be Snizhne sifting through the rubble of an apartment block. Among them are gunmen in camouflage gear.
Across the road is a lower building, also badly damaged.
The video shows a woman on the street beside a pile of bags. Identifying herself as a resident of the block that was destroyed, she says: "Snizhne was bombed this morning and my home was bombed to bits."
Accusing Ukrainian forces of attacking their own people, she calls on Russian President Vladimir Putin to send in troops to defend them.
Ukrainian Security Council spokesman Andriy Lysenko confirmed that the town had been bombed, with damage and casualties.
But he told reporters in Kiev that the town had been attacked by an unknown aircraft with the aim of discrediting Ukrainian government forces.
Ukrainian defence analyst Dmitry Tymchuk argued on Facebook that the attack could only have been carried out by Russian jets as Ukrainian planes had carried out no flights since the An-26 transport plane was downed on Monday.
It would be the first known occasion when a Russian warplane had become involved in the conflict, if true.
In Monday's incident, Ukrainian officials said the An-26 had been hit at an altitude of 6,500m (21,325ft) and must have been targeted with "a more powerful missile" than a shoulder-carried missile, "probably fired" from Russia.
When fully loaded, the plane can fly at 7,600m.
However, defence analyst Charles Heyman, who edits a book called Armed Forces of the European Union, questioned the likelihood of the plane flying at high altitude.
He told the Associated Press news agency the missile was more likely to have been fired by rebels.
"I doubt the transport plane was flying at 6,500m," he said. "That doesn't make sense. The higher you fly, the more it costs, and the plane would have had to be pressurised. It was probably shot down using Sam-6 missiles owned by the rebels, which they have quite a few of."
Nato sources have told BBC defence correspondent Jonathan Marcus that they are increasingly alarmed at the quantity of sophisticated weaponry crossing the border and being gathered on the Russian side.
The Sam-6 is a sophisticated weapons system with impressive radars, our correspondent notes.
Mr Lysenko said two of the plane's crew who survived appeared to have been captured by the rebels. Other members of the crew who did not manage to parachute to safety are believed to have been killed, with human remains found in the wreckage.
Rockets struck Luhansk's southern suburb of Mirny on Monday, setting cars on fire in a car park, as an unverified video posted on YouTube appears to show.
Both government and rebel forces are known to have multiple-rocket launchers in the region.