The United States has assembled one of its mightiest flotillas ever sent to Mideast waters, in the event President Carter decides to take military action against Iran.

The warships are sailing as three separate task forces in waters south and west of Iran, and could bomb or blockade that country.

Pentagon spokesman Thomas B. Ross confirmed that there were 19 ships on station yesterday in the Persian Gulf, Arabian Sea, Red Sea and Indian Ocean.

But he said he would no longer discuss whether the warships, or any other American military forces, were on alert or preparing to be.

A Navy spokesman said yesterday that no one could remember a bigger U.S. naval force in the Indian Ocean area since World War II.

President Carter, who last week warned Iran that under the U.N. Charter the United States could go beyond diplomatic efforts if the 49 American hostages in Tehran are not released, may discuss military action at his news conference tonight.

In advance of the presidential go or no-go decision, U.S. military leaders have deployed forces that could exercise a number of military options.

Because of the lack of nearby land bases to use as a launching pad for any U.S. military operations against Iran, together with the administration's reluctance to land U.S. troops on Iranian soil, naval forces are the president's big stick for this crisis.

The flotilla that Carter could order into action includes the aircraft carriers Midway and Kitty Hawk. The Pentagon said the Midway was in the Arabian Sea yesterday, and the Kitty Hawk, after steaming out of the Philippines, was in the Indian Ocean.

Besides committing the warships to protecting those carriers, the United States has positioned other ships, able to form a blockade, in reach of the Persian Gulf.

The Pentagon yesterday gave this rundown of ships within reach of Iran:

Mideast task force: flagship LaSalle, frigate Aylwin and guided missile destroyer Ricketts in the Persian Gulf; frigate Glover in the Indian Ocean, guided missile frigate Furer in the Red Sea.

Midway task force: carrier Midway, nuclear-powered guided missile cruiser Bainbridge, guided missile destroyer Parsons, frigage Stein and oilers Navasota and Mispillion in the Arabian Sea; frigate Knox and supply ship San Jose in the Indian Ocean.

Kitty Hawk task force: carrier Kitty Hawk, guided missile cruiser Jouett, guided missile destroyer Berkeley, frigate Fanning, destroyer David R. Ray and oiler Wabash in the Indian Ocean.

If Iran harms the American hostages, U.S. military officials predicted, President Carter will opt for hitting some of the bombing targets already drawn up for the planes aboard the Midway and the Kitty Hawk.

Sources, while stressing that they do not know how the list ended up after the possibilities were put before the president, said these are targets that have been discussed over the last two weeks:

Oil resources. The oil fields at the head of the Persian Gulf; the refinery at Abadan, the bombing of which would deny finished products to Iran, but not hurt countries such as Japan and West Germany, which takes crude from the oil fields; Khark Island, where crude is pumped into tankers.

Military bases. Esfahan and Shiraz airfields, where sophisticated F14 fighter planes the United States sold the shah are based; naval facilities at Chah Bahar, Bandar Abbas.

Ayatollah Ruhollah Khomeini's retreat at Qom.

Fighter bombers, when loaded with weapons, can only fly about 300 miles from the carrier before heading for home. Midair refueling from tankers on the carriers can extend that range. But targets at the southern end of Iran would be the easiert to hit from carriers in the Perisan Gulf.