Updated

Secretary-General Ban Ki-moon is urging Syria to give U.N. experts "immediate and unfettered access" to investigate serious allegations of chemical weapons use.

U.N. spokesman Martin Nesirky said Friday that U.N. disarmament chief Angela Kane wrote another letter to Syrian authorities on Thursday urging the government to grant access to the U.N. chemical weapons experts without conditions.

Ban appointed a team of chemical weapons experts to investigate allegations of chemical weapons use in Khan al-Assal in Aleppo, and in Homs, where he received the most evidence. But Syria has only given permission for the experts to visit Khan al-Assal.

Nesirky said Ban has asked the head of the Syrian chemical weapons investigation team, Ake Sellstrom, to visit New York for consultations Monday. The experts have not yet been in Syria.