Kiev: Ukraine said on Wednesday that a battalion of Russian soldiers had set up a base in the southeast of the country, as AFP correspondents reported an apparent retreat of government forces around the main rebel-held city of Donetsk.
Ukraine's claim came just days after Kiev captured Russian troops who Moscow said had strayed accidentally across the border.
Security spokesman Andriy Lysenko said the "tactical battalion" had set up a headquarters near the village of Pobeda about 50 kilometres (30 miles) south of Donetsk.
He said five armoured personnel carriers and a truck had also entered the town of Amvrosiyivka, 25 kilometres northeast of Pobeda.
Journalists travelling around Donetsk, the main hub in the industrial east, said it appeared Ukrainian troops had lost their stranglehold on the city.
There were no Ukrainian troops or checkpoints on the 100-kilometre stretch of road between the town of Novoazovsk on the Azov Sea and Donetsk.
In the village of Starobesheve, to the southeast of Donestk, rebels discovered dozens of muntion caches and abandoned vehicles, the journalists added.
Poland's prime minister also said on Wednesday there was evidence that regular units of the Russian army were operating in Ukraine.
Moscow on Tuesday admitted some of its troops ended up on Ukrainian soil after Kiev reported capturing 10 Russian paratroopers 20 kilometres away from the border.
Russian President Vladimir Putin said the soldiers had been "patrolling the border", and a defence ministry source alleged they had crossed into Ukraine by accident.