“I remain highly skeptical of Russia’s true intentions, but I believe omitting Assad’s bioweapons from any agreement would represent a gaping hole in the plan and would not adequately protect U.S. national security interests,” Cornyn wrote in a letter to Kerry on Friday. “Assad’s bioweapons, either in his hands or the hands of terrorists, represent a direct security threat to the U.S. and our allies.”

Assad is accused of using chemical weapons against his own people last month, prompting President Obama to call for U.S. military intervention, but the administration is now trying a diplomatic route first.

“Any credible agreement must force the surrender of both Assad’s bioweapons and chemical weapons, and it must achieve their destruction in a way that is workable, effective, timely, and verifiable,” the letter stated.

On Wednesday, Assad signed a legal document saying he would comply with an international ban on using chemical weapons but it had no mention of biological weapons. And negotiations continue on if and how he will turn over his arsenal.

Cornyn said biological weapons, which are also considered weapons of mass destruction, are easier to hide and use, making them an “even graver threat.”

In his letter, Cornyn cited a report from the Center for Strategic and International Studies that said Syria was capable of producing biological bombs containing anthrax and botulism.