Iran is fortifying its defenses, rallying loyalists and threatening a wider, harsher response against the U.S. and its Gulf allies as it prepares for potential American attacks after the midterms and seeks to deter them. Iranian officials have warned publicly and in private conversations with regional officials that the Islamic Republic will respond with further escalation across the Middle East if the U.S. launches new strikes. Satellite images show Iran is strengthening sensitive locations, including nuclear sites and border areas, to make them more resilient to attacks.

The government is also preparing Iranians for renewed war, sending paramilitary forces to read war stories to kindergartners, advertising training in firing shoulder-mounted antiaircraft missiles and holding rallies. Iran has little ability to defend against attacks from advanced U.S. jets, high-altitude bombers or cruise missiles, giving it an incentive to try to deter such attacks with threats. The belligerent tone also serves as a distraction from the deepening economic misery Iranians are experiencing as the U.S. toughens sanctions and pressures airlines not to fly to the country, tightening the blockade.

“The war footing is serving them well for now due to an impasse on many fronts—diplomatically, economically and politically,” said Sanam Vakil, Middle East director at London’s Chatham House think tank. “With a new war, the Islamic Republic could dig itself out of the cul-de-sac it has dug for itself.”In an attempt to deter a potential attack, some Iranian officials have hinted that their forces could conduct land incursions into neighboring countries, according to people familiar with the warnings.

Iran’s top national security official, Mohsen Rezaei, said on Sunday that Tehran could target the United Arab Emirates, which has largely escaped being attacked since late spring, as well as data centers and internet cables across the region.

Tehran has separately warned Arab officials that it could strike regional targets including Saudi oil facilities in the event of a renewed conflict, some of the people said. That would add to the threat Saudi Arabia faces from Iran-backed Houthi militants in Yemen, who attacked its capital’s international airport over the weekend. The U.S. military has itself been gearing up for a possible return to fighting. A third aircraft carrier strike group and another Marine expeditionary unit are steaming toward the Middle East, and the Pentagon is weighing sending additional air-defense equipment and munitions to bases in the region, The Wall Street Journal has reported.

While President Trump last week ruled out an attack before the Nov. 3 midterm elections, he has publicly mulled hitting Iran’s Pickaxe Mountain nuclear site, and U.S. officials have said that the military is preparing a range of options for new strikes. The efforts by both sides to shore up their fighting capacity underscore how, more than seven months into the war, neither side feels compelled to give up a fight that is largely deadlocked. Instead, both are trying to break the logjam in their favor.

The U.S. has ramped up its economic pressure campaign and is guiding tankers through the Strait of Hormuz with increasing success. Iran has stepped up its strikes on vessels in the Persian Gulf area. At least three commercial vessels were hit over the weekend in and around the strait, according to U.K. Maritime Trade Operations, which is affiliated with the Royal Navy. A drone attack set a fire on Emirati tanker Petro Dolphin near Oman on Sunday, leaving two Indian sailors missing. The missile attack by Houthi militants on the airport in Riyadh killed at least 13 people and injured hundreds more on Saturday, the single deadliest attack on the U.S. or its Gulf allies in the conflicts related to the Iran war.

The attacks highlighted the vulnerability of major economic targets in the region if Trump decides to resume fighting. The government has also told mediators that it would more aggressively target U.S. personnel, including by trying to sink U.S. warships in the Middle East and kill their crews, according to some of the people familiar with the warnings. Iranian missiles came close to American warships during attacks in September. The Pentagon moved a dozen B-1 bomber aircraft from a U.K. military base last week over concerns that Iran’s Islamic Revolutionary Guard Corps had planned a drone attack.

Direct fighting between the U.S. and Iran has largely been on hold since Trump called off plans for a major escalation in late July, partly because of concerns about supplies of missile interceptors needed to defend targets across the region, the Journal has reported. Iran has used the pause to rebuild its missile and drone facilities and replenish its arsenal. Iran’s supreme leader, Mojtaba Khamenei, praised the work but acknowledged supply-chain struggles in early September comments published by news outlets linked to the Revolutionary Guard.

Iran has also reinforced protection around its nuclear sites and other security-related bunkers. Last month, it built passive defense measures such as makeshift dirt mounds around its Pickaxe Mountain and Natanz nuclear sites to impede any attempted raid by ground forces, according to Vantor satellite images analyzed by the Institute for Science and International Security, a Washington, D.C.-based nonprofit. Iranian officials are touting their efforts. The country’s airports, seaports and power plants “have prepared for wartime conditions,” installing shelters and backup supplies to continue operating in a crisis, Brig. Gen. Gholamreza Jalali, the official in charge of the country’s contingency plans, told reporters at the end of September. State media has also been broadcasting daily pro-regime gatherings.

Iranian residents say the crowds have grown bigger and spread to more locations in recent weeks.A hard-line cleric who joined the demonstrations said he expected the war to restart and Iran to win it. Diplomacy hasn’t worked, the cleric said in an interview, so Iran has no other choice but to fight. Billboards in Tehran now advertise civilian training courses in the use of shoulder-fired antiaircraft missiles and sniper rifles. Last month, the Revolutionary Guard boasted that it had trained its first female commando units in guerrilla tactics, firearms use and drone piloting, according to footage posted by the Fatehin, an elite unit of Iran’s Basij volunteer defense forces.

In Sabzevar, a city in northeastern Iran, a Revolutionary Guard unit visited a kindergarten and told the children stories about the fighting earlier this year and displayed a U.S. Tomahawk missile that the unit said it had neutralized, according to the social-media account of the guard’s ground forces.“In both word and deed, the Islamic Republic is actively preparing for the next war,” said Behnam Ben Taleblu, senior director of the Iran program at the Foundation for Defense of Democracies, a Washington think tank.

“In this sense the regime’s measures constitute both deterrence and war planning.”Write to Benoit Faucon at benoit.faucon@wsj.com