Iran's foreign minister said on Monday the Islamic republic is ready for both war and talks after repeated threats from US President Donald Trump to intervene militarily over a crackdown on protests that activists fear has killed hundreds.

Over two weeks of protests initially sparked by economic grievances have turned into one of the biggest challenges yet to the theocratic system that has ruled Iran since the 1979 Islamic revolution that ousted the shah.

In a sign of the severity of the crisis, the authorities have imposed an internet blackout that has now lasted more than three-and-a-half days and that activists say is aimed at masking the extent of the deadly crackdown.

Seeking to regain the initiative, the government was looking to fill streets nationwide with rallies backing the Islamic republic.

Trump said Sunday that Iran's leadership under Ayatollah Ali Khamenei, in power since 1989 and now 86, had called him seeking "to negotiate" after he repeatedly threatened to intervene militarily if Tehran killed protesters.