Ukrainian troops have advanced up to 30 km into Russia in possibly the most serious border incursion of the conflict. Ukraine's forces stormed into Russia's southwestern Kursk region, deploying around 1,000 troops and more than two dozen armoured vehicles and tanks, according to the Russian army. Andriy Dubchak, reporter at Radio Free Europe, told NDTV the Ukrainian forces will try to hold on to new territory. "Russia will try to do its best to retake the land. It will be hard for Ukraine troops as Russia uses a lot of airstrikes," Mr Dubchak. "Ukraine has a right to fight inside Russian territory. It is a war. Russia tried to occupy Ukrainian territory," he said.