The head of a U.N. investigative panel on Syria says thousands of evacuees sent to rebel-held Idlib and government-controlled western Aleppo province are likely to be caught in escalating fighting from increasingly radicalized extremist groups.

The chairman of the Independent International Commission of Inquiry told reporters Friday that the siege of Aleppo resulted in the further radicalization of some groups.

Paulo Sergio Pinheiro singles out the militant group Hay'at Tahrir al Sham. According to reports, it formed in late January by uniting the al-Qaida affiliate in Syria, the Fatah al-Sham Front, formerly known as the Nusra Front, with four other groups.

Pinheiro says the extremists' presence in Idlib and western Aleppo "raise serious concerns for escalation of hostilities in those areas which puts at risk the evacuees now living there."