The U.S. says the Syrian government used the nerve agent sarin on two occasions in Aleppo in March and April.

U.S. Ambassador Susan Rice detailed the attacks in a letter to U.N. Secretary-General Ban Ki-moon obtained Friday by The Associated Press.

It said sarin was used in a March 19 attack on the Aleppo suburb of Khan al-Assal and in an April 13 attack on the Aleppo neighborhood of Shaykh Maqsud.

It also said unspecified chemicals, possibly including chemical warfare agents, were used May 14 in an attack on Qasr Abu Samrah and in a May 23 attack on Adra.

The letter said an updated U.S. intelligence assessment concluded "that the Syrian government has used chemical weapons on a small scale against the opposition multiple times in the past year."