Russia’s push to take control of Kyiv, the capital of Ukraine, is going slower than expected, according to a senior U.S. defense official.

The Russian military began their offensive Thursday on three “lines of axes” — one from Belarus south toward Kyiv, one north from Crimea, and one from Russia going into Ukraine along the border. The official said on Friday that while those trajectories largely remain unchanged, there appears to be some unexpected resistance from Ukrainian forces.