U.S. President Donald Trump said on Sunday that Iran's leadership had called seeking "to negotiate" as U.S. considers military options to "protect" protesters in the Islamic Republic.

"The leaders of Iran called" yesterday, Trump told reporters aboard Air Force One, adding that "a meeting is being set up. They want to negotiate."

However, Trump added that "we may have to act before a meeting" as Iran is starting to cross U.S. red lines, citing reports of civilian deaths.

The U.S. president was scheduled to meet with senior advisers on Tuesday to discuss options for Iran, Reuters reported. 

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As protests intensify across Iran, will U.S. eventually intervene?

The Iranian government declared three days of national mourning on Sunday for "martyrs" killed in recent protests, the state broadcaster reported.

The move came amid rising fatalities from two weeks of nationwide unrest, which Iran has blamed on the United States and Israel.

Semi-official Tasnim news agency reported that at least 111 members of Iran's security forces have been killed in clashes since the protests initially erupted over a sharp depreciation of the rial and sweeping subsidy reforms.

In response to the unrest, the government on Sunday called for nationwide protests in support of the Islamic Republic on Monday.

In a televised interview with the state broadcaster IRIB on Sunday, Iranian President Masoud Pezeshkian stated that his government is prepared to address the economic grievances driving the protests, but warned that "rioters" will not be tolerated.

"People have concerns, and we should sit with them and resolve those concerns," Pezeshkian said. "But our higher duty is not to allow a group of rioters to come and destroy the entire society."

Pezeshkian also detailed the government's plans to implement a major subsidy reform program aimed at stabilizing markets, boosting production, and increasing purchasing power, providing an update on its current progress.

Israeli Prime Minister Benjamin Netanyahu said his government is "closely monitoring" the situation in Iran.

"If the current leadership were overthrown, Israel and Iran would once again become faithful partners in building a future of prosperity and peace," Netanyahu said during a weekly cabinet meeting.

UN Secretary-General Antonio Guterres urges the Iranian authorities to exercise maximum restraint and enable access to information in the country, his spokesperson said on Sunday.

"The Secretary-General urges the Iranian authorities to exercise maximum restraint and to refrain from unnecessary or disproportionate use of force," a statement from spokesperson Stephane Dujarric read.

(With input from agencies)