NATO Secretary General Anders Fogh Rasmussen said on Friday that any use of chemical weapons in Syria was "completely unacceptable" and a breach of international law.
His remarks came after the White House announced the Syria government had used chemical weapons against rebels.
"This is indeed a matter of great concern. The international community has made clear that any use of chemical weapons is completely unacceptable and a clear breach of international law," he told reporters at NATO headquarters.
"I welcomed the clear U.S. statement. It is urgent that the Syrian regime should grant access to the United Nations to investigate all reports of chemical weapons use," he said.
The White House concluded in a statement Thursday that the Syrian government had used chemical weapons against rebels, a major move that signals deeper U.S. involvement in the Syrian conflict.
Rasmussen said he still believed a political solution was the right way forward in Syria and urged all parties involved, the Syrian government and the opposition to attend an international conference proposed by the United States and Russia jointly.