Kiev called an emergency security session, while a top rebel leader acknowledged today that Russian servicemen are fighting in eastern Ukraine.
The detention of the Russian paratroopers, who claim to have crossed the border accidentally, comes as the presidents of Russia and Ukraine attend a regional summit, with some speculating that the two may meet.
Russian trucks loaded with humanitarian supplies had been idling at the border, but some have now entered Ukraine without permission, which Kiev considers an act of aggression. Russian and Ukrainian leaders are to meet next week.
The Ukrainian government says its troops have made headway in rebel-held Luhansk and Donetsk. Meanwhile, several rebel leaders have left their posts.
Ukrainian and Red Cross officials are inspecting the contents of the 200-plus truck convoy, which Kiev fears could be carrying supplies or troops to bolster rebels.
A fleet of 280 Russian trucks allegedly carrying humanitarian aid is heading for eastern Ukraine, but the Ukrainian government and Red Cross say this isn't part of the agreement they reached.
Russia's ban on food and agricultural imports comes in retaliation for US and European sanctions over Ukraine. Russia is expected to release a full list of the products it is barring today.
At a UN Security Council meeting, Russia raised concerns over displacement in eastern Ukraine and called for greater aid to civilians. Critics say Russia is cloaking its true intentions in a conflict that it helped seed.