Troops from the United States and Ukraine have officially opened joint training exercises intended to help bolster Ukraine's defenses against incursions from Russian-backed separatists in the east.

Speaking under driving rain at a military base in the western region of Lviv, Ukrainian President Petro Poroshenko said the armed forces needed to be rebuilt from scratch to deter international military threats.

The exercises have drawn enraged reactions from Russia, which has described them as a potential cause of destabilization. Moscow continues to dismiss mounting evidence of its involvement in fomenting and supporting a separatist insurgency in Ukraine that has claimed more than 6,000 lives over the past year.

The 300 U.S. Army paratroopers involved in the training traveled to Ukraine last week and will be working alongside 900 national guardsmen.