In her final press interview as US Secretary of State, Mrs Clinton said one of Iran's "highest priorities" was keeping Syrian President Bashar al-Assad in power.

"We believe they have acted on that by sending in more personnel, not only to help Assad, but to support and advise military security forces," she told reporters.

Mrs Clinton added that Iran had increased the quality of its arms sent to Syria because "Assad is using up his weaponry".

She expressed similar concerns with regards to Russia's involvement in the conflict.

"We have reason to believe that the Russians continue to supply financial and military assistance in the form of equipment," she said