Earlier in the day, Russian Foreign Minister Sergei Lavrov said that the United States appears to have a wish to maintain al-Nusra Front terrorist group in some form in Syria, and use it to achieve its own goals.
"Al-Nusra is a designated terrorist organization, is not a party to the cessation of hostilities and certainly not a part of the political future of Syria. [US] airstrikes against this group will continue," Kirby stated.
The ceasefire is too fragile, Kirby claimed, because of violations committed by the Syrian government and al-Nusra Front.
The ceasefire in Syria worked out by Russia and the United States took effect on February 27. The cessation of hostilities does not apply to terrorist organizations, such as Daesh and al-Nusra Front, outlawed in a number of countries worldwide including Russia and the United States.
Syria has been mired in civil war since March 2011, with government forces loyal to Syrian President Bashar Assad fighting numerous opposition factions and extremist groups.