WASHINGTON, July 22 (Reuters) - U.S. President Donald Trump will attend a ceremony on Wednesday to bring back four service members to the United States in flag-draped coffins after they were killed in Iranian attacks in the Middle East.

These ceremonies, known as the "dignified transfer" of the bodies, come amid growing frustration over Trump's war on Iran.

The war has cost American taxpayers at least $37.5 billion so far, and has resulted in the deaths of 18 American soldiers and injuries to more than 450 American troops, in addition to the deaths of thousands of Iranians.

Lawmakers from both the Republican and Democratic parties have raised questions about the oil price war, which has caused global oil prices to soar and continues with no end in sight. Trump has outlined conflicting objectives for the war since it began in February.

The US president will travel to Dover Air Force Base in Delaware to attend the ceremony.

The presidents of the United States, their deputies, and dignitaries usually attend the official ceremonies for the transfer of bodies at Dover, home of the largest U.S. military mortuary, during times of war or conflict that result in the deaths of American soldiers.

Three American service members—a man and two women—were killed when Iran targeted their sleeping quarters at Muwaffaq Salti Air Base in Jordan with a missile on Friday. The deceased were Lieutenant Tyler James Vian, 25; Sergeant Angel S. Rampersad, 28; and Private Isabella Gonzalez, 19. All three were members of the U.S. Army Air Defense Brigade.

The fourth, Sergeant Michael Emmanuel Swinton, 30, was killed in a separate incident on Sunday in Erbil, Iraq, during a controlled detonation of an attack drone.

The U.S. Department of Defense (Pentagon) said on Monday that 100 military personnel have been injured since July 7, and that 96 percent of them have returned to duty.

A Reuters/Ipsos poll conducted earlier this month showed that four out of five Americans expect a prolonged U.S. war with Iran. About 37 percent of those polled supported Washington launching military strikes against the Islamic Republic.

Trump's approval rating has remained near its lowest point since the start of the conflict. Republican strategists said that rising living costs have negated the political benefits of his tax cuts.

High gas prices and cost-of-living concerns pose a political risk to Trump's Republican Party ahead of the November midterm elections, in which the party risks losing its majority in the House of Representatives and possibly its control of the Senate.