"Considering [the Guards'] full knowledge about the centers of deployment of the criminal terrorists' leaders., they will face a deadly and unforgettable vengeance in the near future", the IRGC said in a statement carried by state media. It is Americans who are provoking them.
He said: "It is America who supports these little mercenary countries in the region".
"The attacks are doubtlessly meant to tarnish the prestige of the IRGC, but I believe the terrorist incidents will strengthen the IRGC's standing and even mobilise some public support", said Ali Alfoneh, senior fellow at the Gulf Arab States Institute in Washington.
"America is acting like a bully towards the rest of the world. and thinks it can act based on brute force", said Rouhani.
"The government is ready to counter any action by the United States, and the Americans will regret this".
Since the beginning of the year, Iranians have kept protesting and marching against the clerical regime, and the national currency has lost about two-thirds of its value, said Maryam Rajavi, leader of the People's Mujahedin of Iran, or MEK, and the declared president-elect in exile of the National Council of Resistance of Iran.
"Hopefully we will overcome these sanctions with the least possible costs and make America regret its aggressiveness towards other countries, and particularly Iran", said Mr Rouhani.
The British charge d'affaires "was told that it is not acceptable that the spokesman for the mercenary Al-Ahwazi group be allowed to claim responsiblity for this terrorist act through a London-based TV network", said the ministry's spokesman, Bahram Ghasemi.
"It is obvious for us that they [the terrorists] are affiliated to what group".
The United Kingdom's ambassador to Iran Rob Macaire tweeted his condolences.
Ms. Haley said Iranian President Hassan Rouhani had "oppressed his people for a long time", BBC reported on Sunday.
Rouhani's comments blaming the U.S. came as Iran's Foreign Ministry summoned diplomats from Britain, Denmark and the Netherlands over them allegedly providing havens for the separatists. "The thing he's got to do is look at the mirror", the U.S. envoy to the UN, Nikki Haley told CNN's 'State of the Union, ' referring to Rouhani.
There was no immediate Gulf Arab comment on his remarks.
The semi-official Tasnim News Agency, first reported the casualties and included members of Iran's Revolutionary Guard Corps among the victims.
The Revolutionary Guards are the most powerful military force in the country and answer to Khamenei.
The U.S. State Department has said Giuliani does not speak for the administration on Iran.
On the same day Arab separatists killed at least 25 people in an attack targeting a military parade in southwestern Iran, President Donald Trump's lawyer mounted a stage in NY to declare that the government would be toppled.
Regional governments condemned the attack and offered their condolences, including Hamas and Qatar and a number of Saudi Arabia's reginal allies such as Kuwait, Oman and Egypt. Saudi Arabia was missing from the list of customary diplomatic condolences.
Trump's own actions and the close coordination we have seen between his administration, Saudi Arabia, UAE and Israel on Iran raises the prospects of a different explanation: one in which the United States itself is actively pushing its allies and being pushed by its allies towards war with Iran.
Iranian security forces shot and killed three of the terrorists and arrested a fourth, according to the news agencies.