United States officials are bracing for Iran to respond to the killing of General Qassem Soleimani, and are preparing for a possible "tit-for-tat" attempt on the life of an American military commander.
Meanwhile, the leader of Iran's Revolutionary Guard, Hossein Salami, has threatened to "set ablaze" places supported by the United States over the killing of a top Iranian general in a US air strike last week.
Mr Salami made the pledge before a crowd of thousands gathered in a central square in Kerman, the home town of General Soleimani.
The leader of Iran's Revolutionary Guard praised Gen Soleimani's exploits and said as a martyr, he represented an even greater threat to Iran's enemies.
"We will take revenge," he said.
Iran's parliament, meanwhile, passed an urgent bill declaring the US military's command at the Pentagon in Washington and those acting on its behalf "terrorists", subject to Iranian sanctions.
President Donald Trump ordered last week's strike against General Qassem Soleimani, the head of Iran's elite Quds Force, after the death of an American contractor in Iraq.
The Iranian commander is thought to have played a major role in the deaths of thousands of civilains and western military personnel, while the US said he was "actively developing plans" to attack Americans in Iraq and the Middle East.
Mourners in Kerman dressed in black carried posters bearing the image of General Soleimani, a man whose death prompted Iran's supreme leader to weep over his coffin on Monday as a crowd said by police to be in the millions filled Tehran streets.
The outpouring of grief was an unprecedented honour for a man viewed by Iranians as a national hero for his work leading the Guard's expeditionary Quds Force.
US officials believe the next steps by Iran will determine the ultimate course of the latest crisis.
The killing of General Soleimani has already pushed Tehran to abandon the remaining limits to its 2015 nuclear deal.
Meanwhile, pro-Iranian factions in the Iraqi Parliament have pushed to oust American troops following General Soleimani's killing on Iraqi soil.
US officials say American intelligence is not clear on whether Iran's latest military moves are designed to bolster Tehran's defences or prepare for an offensive strike, but the US is continuing to reinforce its own positions in the region, including repositioning some forces.
One official said the US anticipated a "major" attack of some type within the next day or two.
On Monday, Defence Secretary Mark Esper said no decision had been made about withdrawing troops from Iraq.
Tensions between the US and Iran have been regular since Mr Trump began his "maximum pressure" campaign against Iran shortly after taking office.
He abrogated the 2015 nuclear deal and reimposed crushing economic sanctions.
Both steps were aimed at preventing Iran from getting a nuclear weapon and deterring the sort of regional aggression spearheaded by General Soleimani.
Mr Trump has repeatedly contended he is not seeking "regime change" in Iran, a move promoted by some of his advisers.
The US military has increased the protection of its forces, particularly in Iraq.
Officials said a number of the recently deployed soldiers from the 1st Brigade of the 82nd Airborne Division had moved into Iraq from Kuwait in order to increase security for Americans there.
The US military has also suspended all training of Iraqi forces to focus on force protection.
The US military's concern about its vulnerability to Iranian attack in the Persian Gulf region has been at a heightened state since about May, when the administration reported it was getting intelligence indications that Iran was planning attacks on US interests in Iraq and elsewhere in the region.
The Pentagon sent additional forces to the Gulf at that point, and in July it worked out an arrangement with the government of Saudi Arabia to send US forces to a large base deep in the Saudi desert, in less obvious range of Iranian missiles.
The main hub for American military air operations throughout the Middle East is located at al-Udeid air base in Qatar, within easy range of Iranian missiles.
American forces also are stationed in the United Arab Emirates and Kuwait.
The Navy's 5th Fleet, which operates throughout the region, is based at Bahrain.
Meanwhile, in Washington, Congress, which has the sole power to declare war, has complained that Mr Trump did not provide advance notice of his air strike in Baghdad.
Mr Trump did meet the 48-hour deadline required by the War Powers Act to notify Congress after the deadly drone strike. The document was classified, and no public version was released.
Cover image: Iranian mourners during General Qassem Soleimani's funeral in Tehran (Picture: PA).