Two volleys of Grad rocket fire rang out in Ukraine's eastern, rebel-held city of Donetsk, underscoring the difficulties of enforcing a cease-fire almost a week after it was signed.

The city council of Donetsk confirmed in a statement Thursday there had been multiple explosions during the morning, but reported no casualties.

Col. Andriy Lysenko, spokesman for Ukraine's National Security and Defense Council, told journalists that a large swath of the border near the Sea of Azov had come under full rebel control. While the changes likely happened before the cease-fire, it was an unusual admission by Ukraine of the scale of a rebel counteroffensive that Kiev says was backed by Russia.

The cease-fire between Russian-backed separatists and Ukrainian troops was imposed late Friday but has been riddled by violations.