Pro-Russian separatists have opened fire on unarmed Ukrainian soldiers as they were crossing back into Ukraine from Russia where they had taken shelter from fighting, Ukrainian defence officials say.
The soldiers made their return on Tuesday, the day after Ukraine acknowledged that 311 soldiers and border guards had been forced by fighting with separatists to cross into Russia.
It said they had destroyed their weapons before crossing the border, but the rebels said they had left them behind, enabling separatists to seize them.
"Today at about 6am [separatist] fighters treacherously fired on a column in which there were 195 unarmed Ukrainian troops returning from the Russian Federation," Andriy Lysenko, a spokesman for the national defence council, told Interfax news agency.
He had no immediate word on casualties.
The soldiers crossed the border on Monday, leading some reports that they had "abandoned Kiev".
Vasily Malayev, head of the Federal Security Service's border patrol in the Rostov region said the Russian side had opened a safe corridor for the soldiers into Russia.
Late on Monday, Malayev said 180 of the troops had since been returned to Ukraine after consultations with Kiev while the rest were deciding what to do next.
Government troops have been battling the rebels in the Russian-speaking east since April.
The UN High Commissioner for Human Rights says more than 1,100 people including government forces, rebels and civilians have died in the conflict.
An additional 117,000 people have been internally displaced, UNHCR said.
As the Ukrainian army recaptures more territory from the separatists, Russia alarmed Ukraine on Monday by announcing military exercises near the long joint border.
More than 100 aircraft, including fighter jets and bombers, would take part in the manoeuvres this week in the central and western military districts, the Russian air force said.
The Ukrainian Foreign Ministry condemned the Russian move on Tuesday and accused Russia of continuing to arm the rebels.
"We categorically demand from the Russian side to put an end quickly to such criminal actions (and) to pull its troops back from the border with Ukraine," it said in a statement.
Ukrainian security officials said separatist fighters were launching counter-attacks to break a tightening noose around their positions while Ukrainians troops continued to come under mortar and artillery shelling from inside Russian territory.
Russian military equipment and consignments of arms continued to roll into Ukraine to supply the rebels, the Ukrainians said on Tuesday, though Russia denied it is providing support for the rebels who seek to set up pro-Russian 'people's republics' in the east of Ukraine.