Iran's foreign minister is insisting that use of chemical weapons in Syria would also be a "red line" for Tehran, but suggests rebel forces should be investigated rather than the Islamic Republic's allies in Damascus.

Ali Akbar Salehi's comments reflect the deep divide between Tehran and the West over Syria's civil war. Iran remains loyal to Syrian President Bashar Assad despite recent suspicions that his forces used chemical agents.

The semiofficial ISNA news agency quotes Salehi as saying Tuesday that the use of chemical weapons "by anybody is our red line." But he claimed opposition groups could have used chemical agents and demanded a U.N. probe.

Last week, Washington said it had "varying degrees of confidence" that pro-Assad forces used small amounts of the nerve agent sarin against rebels.