The president’s declaration follows Ukrainian charges that columns of Russian military vehicles crossed into Ukraine three days ago to open up a new front in the fighting in southeastern Ukraine, where rebels appear to have firmly taken control of the town of Novoazovsk.

Ukraine also said this week that it had captured 10 Russian paratroopers who had crossed into Ukraine, and showed video of some of the men being interviewed. Putin denied the allegations, suggesting the soldiers apparently crossed the unmarked border by accident while on training exercises.

Russian markets dived as fears grew that the country was escalating its role in the conflict, a move that could provoke the U.S. and European Union to impose further sanctions on Russian businesses and individuals. Russia’s MICEX index dropped nearly 2 percent on Thursday, and major Russian banks VTB and Sberbank dropped more than 4 percent.

On Thursday morning, an Associated Press journalist saw rebel checkpoints at the outskirts of Novoazovsk and was told he could not enter. One of the rebels said there was no fighting in the town.