The Pentagon warned Russia on Tuesday against interfering with the site of an alleged chemical weapons attack in Syria's regime-held city of Aleppo.

The Pentagon said Assad may try to interfere with the site of the incident and build a narrative to justify attacking the rebel-held stronghold of Idlib, which is currently protected under a 10-week-old truce deal in northern Syria.

"It is essential to ensure that the Syrian regime does not seize on false pretexts to undermine this ceasefire and launch an offensive in Idlib," Pentagon spokesman Commander Sean Robertson said in a statement.

"We caution Russia against tampering with another suspected chemical weapons attack site and urge Russia to secure the safety of the OPCW inspectors so these allegations can be investigated in a fair and transparent manner."

"We urge immediate inspection of the alleged site by international investigators, with freedom to interview all involved and unhindered ability to collect evidence," Robertson said.