The secretary of Ukraine's National Security and Defense Council, Andriy Parubiy, submitted his resignation on Thursday.
Ukrainian President Petro Poroshenko accepted the resignation of the country's security chief and thanked Parubiy for his work "in the darkest days of the existence of our country."
"Every day Andriy Parubiy in his work focused on improving the defense of our country."
Upon handing in his resignation, Parubiy thanked the president and his NSDC staff for their work "and extremely important decisions that helped us to survive."
It is unclear why Parubiy has chosen to step down. On his Facebook page, Parubiy wrote, "I believe it is unacceptable to comment on my resignation in a time of war," adding that "I will continue to assist the front, primarily volunteer battalions."
Parubiy's resignation comes as the Ukrainian military is fighting to regain control of the pro-Russian rebel stronghold of Donetsk in restive eastern Ukraine.