US President George W Bush has called Iranian-backed "Shi'ite extremists" as great a peril as al Qaeda - singling out Lebanon's Hezbollah rather than the Shi'ite Islamist factions empowered by his war in Iraq.
There was no immediate official reaction from Tehran or from Hezbollah, but one Iranian analyst said Mr Bush was "softening up" his critics in Congress for a possible strike on Iran's nuclear facilities.
Iran says the programme is civilian not military.
Hermidas Barvand, a Tehran university professor, said Mr Bush sought to "magnify the menace of Shi'ites" for two reasons: "to mobilise Sunni Arabs ... and to legitimise future measures by creating a resemblance between Shi'ite extremism and al Qaeda".
The President's broadside, in his annual State of the Union speech, was in line with his "for us or against us" division of the Middle East since the 9/11 attacks on the United States.
He has opted to isolate and confront Iran and Syria, along with the political-military Islamist groups Hezbollah and the ruling Palestinian Hamas, rather than talking to them.
Both states have tried this month to avoid isolation and show they cannot be ignored in a region of intertwined conflicts.
Syria has hosted Iraqi President Jalal Talabani and brokered talks in Damascus between Hamas and its Fatah rivals.
Shi'ite Iran has reached out to Saudi Arabia, US ally and bastion of Sunni Islam, in an apparent effort to keep sectarian warfare in Iraq from igniting in Lebanon and beyond.
"Some of the contacts between Saudi Arabia and Iran are very significant," Paul Salem, director of the Middle East Centre of the Carnegie Endowment for International Peace, said.
He said Tehran and Riyadh feared that Sunni-Shi'ite strife could spin out of control and wanted to ensure that "the brush fire that exists in Iraq doesn't become a forest fire that could consume Lebanon, Syria, Palestine and parts of Saudi Arabia".
Mr Bush's emphasis on Shi'ite radicals seemed designed in part to please US-backed Sunni rulers in Saudi Arabia, Egypt and Jordan, who are alarmed by rising Shi'ite power and Iranian influence in Iraq since the 2003 invasion.
Mr Bush said Sunni militants such as Abu Musab al-Zarqawi, who was killed in Iraq, and al Qaeda leader Osama bin Laden were just one part of a "totalitarian" threat from Islamist radicals.