Ukraine is accusing Russia of launching a new military incursion across its eastern border.
Kiev says a group of Russian soldiers have crossed the border and entered a town not far from where Ukrainian authorities detained 10 Russian soldiers on Monday.
Moscow has yet to comment on the new accusations, but contends the soldiers captured on Monday inadvertently crossed an unmarked section of the border by mistake.
Meanwhile, separatists in eastern Ukraine have pushed their way into the town of NoVOAzovsk, threatening to open up a new front in the conflict.
The town lies in a strategically significant location - on the Azov Sea and on the road linking Russia to the Russian-controlled Crimean Peninsula.
Andriy Lysenko is a spokesperson for Ukraine's National Security Council.
"NoVOAzovsk town is under shelling from the Russian side, but the town is under control by the Ukrainian army. Mariupol city is supported by extra army units."
The latest moves come just hours after Ukrainian President Petro Poroshenko promised a "roadmap" for peace in the country's east after holding direct talks with Russian President Vladimir Putin in Belarus.
