Thousands gathered in Baghdad Saturday to mourn the death of those killed in a U.S. airstrike that has sent tensions soaring throughout the Middle East.

Mourners chanted "Death to America, death to Israel" as they marched in a funeral procession for Iran's top general and Iraqi militant leaders who were killed in the strike early Friday near the capital's international airport.

Among them was Iraq's prime minister, Adel Abdul-Mahdi.

The crowd of mostly men dressed in black fatigues carried their country's flag and the flags of Iran-backed militias that are loyal to Qassem Soleimani, the high-profile commander of Iran's secretive Quds Force whose death has raised fears of escalation and even all-out war.

Mourners surround a car carrying the coffin of Iranian military commander Qassem Soleimani during a funeral procession for him and nine others in Baghdad Saturday. AHMAD AL-RUBAYE / AFP - Getty Images

President Donald Trump said Friday that he ordered the the drone attack that killed Soleimani in order to prevent a conflict.

Congress received formal White House notification of the attack Saturday, according to two senior Congressional sources. The notification is required within 48 hours under the War Powers Act of 1973.

It was sent to House Speaker Nancy Pelosi, D-California, and Sen. Chuck Grassley, R-Iowa, the Senate's president pro tempore. The document was classified, the sources said.

Iran has vowed revenge and "harsh retaliation."

On Saturday, one rocket landed inside the Green Zone, a secured area in central Baghdad where the U.S. and other nations have embassies, and another in the Jadiriyah neighborhood across the Tigris from the Green Zone, a U.S. military official and Iraq's Joint Operations Command confirmed.

Three rockets were also fired at Balad Air Base, an Iraqi Air Force compound nearly 50 miles north of Baghdad. All three projectiles landed outside the installation where U.S. personnel are based, a U.S. military official said.

There were no reports of casualties or damage to buildings. NBC News has not confirmed the source of the rockets.

Washington said Friday it was dispatching another 3,000 troops to the region, while U.S. allies across the world have urged caution.

Qatar, which often serves as a mediator in the Middle East, said its foreign minister had discussed "ways of exercising restraint" during a visit to Tehran Saturday.

As the world waits to see what comes next, Iraq's Popular Mobilization Units (PMU) said early Saturday another airstrike hit a convoy north of the capital, killing at least six people.

But both the Iraqi military and the PMU — an umbrella group of mostly Iran-backed militias — later denied any airstrikes had taken place in the area. The U.S.-led coalition also denied carrying out any airstrike.

Abu Mahdi al-Muhandis, said to be the deputy of the militias and a close adviser to Soleimani, was also killed in Friday's strike.

Iraq's president, Barham Saleh, told NBC News Friday that tensions are running high in the country, which has been gripped by anti-government demonstrations and saw protesters storm the U.S. embassy in Iraq earlier this week.

"The situation in Iraq is very fragile, very precarious," he said, calling for restraint on all sides.

"Iraq cannot be condemned to another cycle of violence. We have had too many wars over the last four decades."

The Iraqi Parliament plans to meet Sunday to discuss how the nation will respond.

The Pentagon has justified the targeted killing of Soleimani by saying he was actively developing plans to attack U.S. diplomats and service members in Iraq and elsewhere throughout the region.

He is also blamed for orchestrating a series of attacks on allied bases in Iraq in recent months, including a rocket strike that killed a U.S. contractor and wounded four other service members last week.

That prompted the U.S. to carry out deadly airstrikes last Sunday on weapons depots in Iraq and Syria that it said were linked to Kataeb Hezbollah, an Iran-backed Shiite militia group. That in turn prompted the demonstrations at the U.S. Embassy involving as many as 6,000 people, many of whom were linked to the group.

Iraq's leadership is facing mounting pressure to expel the 5,200 American troops it hosts. The country's parliament is expected to hold an emergency session on Sunday in response to the airstrike.

Meanwhile in Iran, President Hassan Rouhani visited Soleimani's house to pay respects to the late general's family. He was seen hugging Soleimani's son and greeting his wife.

Images of the general have been plastered across billboards around the capital, where a farewell ceremony will be held on Sunday evening in Tehran's Mosalla Mosque.

The main funeral ceremony will be held in Tehran on Monday where Khamenei will lead prayers. On Tuesday, Soleimani will be buried in his home town of Kerman.

"They (Americans) do not realize what a big mistake they have made," Rouhani warned as he visited the family.

"They will see the consequences of their mistake not only today but in the years to come."