Iran is fully prepared for conflict but also ready for negotiations, its foreign minister said on Monday, as U.S. President Donald Trump said the Iranian leadership was seeking talks after he threatened to intervene militarily over a crackdown on protests.

U.S. President Donald Trump has said that Iran's leadership had called him seeking "to negotiate" after he repeatedly threatened to intervene militarily if Tehran killed protesters.

A channel of communication is open between Iran and U.S. President Donald Trump's special envoy for the Middle East despite the lack of diplomatic relations, the Iranian foreign ministry spokesman said Monday.

At least 192 people have been killed in two weeks of protests against the government and economic strain in Iran, a rights group said on Sunday, in a sharp rise from an earlier death toll of 51.

Iranians took to the streets in new protests against the clerical authorities overnight despite an internet shutdown, as rights groups warned on Sunday that authorities were committing a "massacre" to quell the demonstrations.

Major Iranian cities were gripped overnight by new mass rallies denouncing the Islamic republic, as the son of the ousted shah urged protesters on Saturday to plan to seize city centers.

Venezuela was set to hold talks on Saturday with U.S. envoys in Caracas on restoring diplomatic ties, days after U.S. forces deposed Nicolas Maduro as its president.

South Africa on Saturday began naval drills with Russia, Iran and China, describing the manoeuvres off its coast as not merely a show of force but a vital response to rising maritime tensions.

Iran's supreme leader Ayatollah Ali Khamenei on Friday insisted that the Islamic republic would "not back down" in the face of protesters who he called "vandals" and "saboteurs", in a speech broadcast on state TV.

Speaking to supporters in his first comments on the escalating protests since January 3, Khamenei said U.S. President Donald Trump's hands "are stained with the blood of more than a thousand Iranians" and predicted the "arrogant" U.S. leader would be "overthrown" like the imperial dynasty that ruled Iran up to the 1979 revolution.

Russia on Friday said the United States had decided to release two Russian members of the crew of a Russian-flagged oil tanker that Washington seized earlier this week.

"In response to our request, U.S. President Donald Trump has decided to release two Russian citizens aboard the Marinera tanker, previously detained by the United States during an operation in the North Atlantic," Russian foreign ministry spokeswoman Maria Zakharova said in a statement.