The top US diplomat has claimed, without evidence, that the Islamist militant group has established a new base in Iran.

US Secretary of State Mike Pompeo said Tuesday that al-Qaida has established a new home base in Iran, and has "burrowed inside" the country, making it harder for the US to deal with the militant group.

The claim was met by skepticism by members of the US intelligence community and was blasted by Iranian Foreign Minister Mohammad Javad Zarif on Twitter as "warmongering lies."

As a clerical state based on Shiite Islam, Iran is considered ideologically opposed to extremist groups like al-Qaida, which adhere to the Sunni branch of Islam and have traditionally been supported by Iran's arch-enemy Saudi Arabia. 

A former head of the Central Intelligence Agency (CIA), Pompeo has previously said that al-Qaida founder Osama bin Laden "considered al-Qaida members inside Iran to be hostages."

A new al-Qaida-Iran axis?

Pompeo's arguments of Iranian support for al-Qaida marks one of the first times Washington has clearly accused Tehran of supporting a Sunni extremist group. Iran is a known supporter of Shiite militant groups, such as Lebanon's Hezbollah. 

However, Pompeo has said the relationship between Iran and al-Qaida has improved since the 2015 nuclear agreement brokered by the Obama administration. 

US intelligence experts have said Iranian territory provides a relatively safe haven from the US military, and al-Qaida's presence there comes in exchange for guarantees that the extremists will not target Iranian interests.

"Iran decided to allow al-Qaida to establish a new operational headquarters, on the condition that al-Qaida operatives inside abide by the regime's rules governing al-Qaida's stay inside the country,'' Pompeo said.