PARIS, AUG. 11 -- France and Britain announced today that they are sending mine sweepers to protect their freighters and warships in the Persian Gulf region. Last month, the two countries had turned down a U.S. request to do so.

Both governments emphasized that the European mine sweepers will operate separately from a U.S. naval force escorting reflagged Kuwaiti oil tankers through the gulf. The additional ships instead will reinforce British and French flotillas that have been patrolling the area for some time, the French and British defense ministers said in nearly simultaneous announcements.

In Washington, State Department spokesman Charles Redman said, "We certainly welcome those actions as we welcome all efforts to contribute to ensuring freedom of navigation in the gulf . . . . These are decisions that have been taken by other governments in response to . . . dangerous international shipping." The Pentagon similarly praised the actions.

The decisions in London and Paris marked an important symbolic step in the direction of efforts by the Reagan administration to organize an international mine-sweeping force. According to an informed diplomatic source, a chief goal of these efforts has been to enable Washington to portray its gulf operations as part of an allied undertaking as well as to enhance actual mine-sweeping capabilities.

The major European allies, including France and Britain, had rejected U.S. entreaties late last month to join such a force, embarrassing the Reagan administration.

The Netherlands and Italy, which also were approached then, have continued to say they would contribute mine sweepers or other warships only to a multilateral European force under the U.N. flag.

George Younger, the British defense secretary, announced in London that four British mine sweepers and a support vessel are to reach the area in about five weeks.

Explaining why Prime Minister Margaret Thatcher's government appeared to have changed its mind, he said conditions are different since mines were discovered yesterday in the Gulf of Oman at the mouth of the Persian Gulf.

"There is an increased danger from mines in the Armilla patrol's operational area," Younger said, according to news agencies. "The government has therefore decided to equip the Armilla patrol {the name given the British naval force} with a mine-sweeping capability to enable it to continue to carry out its task effectively."

The British force has consisted of a destroyer, two frigates and support craft, according to reports from London. It has been protecting British-flagged merchant shipping in the eastern end of the Persian Gulf for a number of months in deliberately low-profile operations.

Younger said Britain will inform the United States of its military deployment in the gulf, including the mine sweepers. But he said he would make no commitment involving any "hypothetical situation" in which the British mine sweepers could be made available to clear areas where the U.S. naval force is about to pass.

In addition, Younger reiterated that British warships on station in and around the gulf will continue to accompany only British-flagged ships. "There is no change in that at all," he said.

Britain's reversal followed not only the worsening situation in the gulf, but concern in London that the Thatcher government's original refusal to send mine sweepers was portrayed on both sides of the Atlantic as a stunning and uncharacteristic rebuff of urgent administration entreaties.

Thatcher is known to be worried over Reagan's current domestic troubles and anxious for his administration to retain a strong western leadership role during its remaining time in office. During a visit to Washington last month, she offered verbal backing for U.S. policy in the gulf.

Later, when the mine-sweeping request came, Thatcher rejected it, saying that an additional western military presence in the gulf would escalate tension there.

Her government tried to soften the blow with a lengthy diplomatic explanation sent privately to Washington and a promise to keep the issue under consideration in light of changing circumstances in the gulf.

Washington repeatedly pressed London in recent days to reconsider its decision, arguing strongly for the need to show western solidarity and noting that the mining of the gulf was a challenge to British interests.

In explaining France's decision to dispatch two mine sweepers and a support craft to join French naval forces in the area, Defense Minister Andre Giraud said the action was taken because of the mines discovered outside the gulf. He said France was not acting in concert with the United States or Britain.

"We do not foresee carrying out any combined operations," he added in response to questions about possible cooperation with the United States.

Prime Minister Jacques Chirac's government on July 29 dispatched an aircraft carrier group -- the carrier Clemenceau, two missile frigates and an oiler -- to take up position in the Gulf of Oman at the mouth of the Persian Gulf. The force recently passed through the Suez Canal and was reported near a French naval base at Djibouti. It will join France's regular Indian Ocean fleet, including three missile frigates and support vessels. Giraud said the French forces will continue a policy of accompanying French-flag merchant ships only on a case-by-case basis.

Despite both ministers' pains to dissociate their decisions from U.S. gulf operations, it seemed unlikely that U.S., French and British mine sweepers, along with a growing list of other warships, would avoid coordinating their actions once all the vessels arrive in the Persian Gulf region. In that light, the Reagan administration appeared to be gaining the practical effect of an international mine-sweeping force if not the title.

The Reagan administration has insisted that European governments should help keep gulf navigation safe since their countries benefit most from oil shipped through the waterway. Only a small percentage of U.S. oil imports travel through the gulf.

A Foreign Office official, David Mellor, referred to that argument today at a news conference in London, saying Britain would urge other European nations to send mine sweepers to the gulf to protect shipping "of vital importance to the economies of Western Europe."

"We strongly support the idea that other nations should play their part in what is basically keeping the high seas open to peaceful traffic," Younger said.

European governments have been reluctant to associate with the Reagan administration's deployment in the gulf for fear of provoking Iran and of raising tensions in an already dangerous region, diplomats here said.