Earlier in the day, Russian Defense Minister Sergei Shoigu offered the United States to begin work on May 25 on jointly planning and carrying out airstrikes against al-Nusra Front as well as other illegal armed groups in Syria.

"We do not collaborate or coordinate our operation with the Russians. As the Secretary [Carter] has said, Russian operations are supporting and enabling the Assad Regime which is like pouring fuel on a fire. We remain focused solely on degrading and defeating ISIL [Islamic State]," Allen stated.

However, the United States will continue to use the bilateral Memorandum of Understanding (MOU) with Russia on flight safety during combat missions over Syria that has proven to be effective, US Department of Defense spokesman Matthew Allen said.

"Outside of the MOU, we have shared with the Russians rough geographic areas where coalition forces may be operating, and have advised them to remain clear of these areas. To date, they have honored this," Allen stated.