Secretary of State Alexander M. Haig Jr.'s shuttle diplomacy to defuse the Falkland Islands dispute appeared to have run into difficulties today as he postponed his scheduled return to Argentina, after saying that there are still substantial difficulties between Britain and Argentina.

Following 11 hours of talks with British Prime Minister Margaret Thatcher, Haig told reporters last night as he left 10 Downing Street, "We made some progress" toward a solution of the conflict, "but a number of substantial difficulties remain."

He said then that he would be returning immediately to Buenos Aires to discuss these difficulties, "because time is slipping away from us."

But early this morning the U.S. Embassy here announced that Haig was staying in London for the time being. Without further explanation, the embassy said Haig had been in touch several times last night by telephone with Buenos Aires and "a complication has arisen at that end."

Neither British nor U.S. officials would say what the difficulties were nor when Haig would fly back to Buenos Aires. A British government official said another meeting with Thatcher was set for 9 a.m. GMT, United Press International reported.

Earlier in the evening Haig had said, "We made some progress" in detailed discussions with Thatcher and her foreign and defense secretaries of "some ideas we had developed in Buenos Aires" during talks on Saturday with the Argentine military government of President Leopoldo Galtieri.

But, asked if he was more hopeful of avoiding military conflict between Britain and Argentina, Haig answered, "not at all, not at all."

Thatcher government officials said the British task force continues to move toward the Falklands, from which much of it is still more than a week away, and the British threat to attack Argentine warships found in the 200-mile zone around the Falklands remains in effect. They said there currently were no Argentine "targets" within the zone, which is being patrolled by several deeply submerged, nuclear-powered British hunter-killer submarines.

"There is no truce, hesitation or pause in any of the military preparations, as I understand it, that are under way" Haig said when asked if the military situation had yet been affected by his shuttle diplomacy.

In an indication that Argentina may be prepared to send its fleet into the South Atlantic, an Argentine Navy communique, meanwhile, said that Navy Commander Jorge Anaya had visited the major Navy base at Port Belgrano to give a message to the fleet, which is ready to head out "when the command is given," Washington Post correspondent Jackson Diehl reported from Buenos Aires.

Earlier, Argentina's military government endorsed a proposal for a 72-hour truce with Britain while Haig seeks a diplomatic solution to the dispute and called on Britain to suspend its naval blockade in the South Atlantic.

The Argentine statement came in response to a proposal by the Peruvian government as Haig met with British officials in London. Britain is studying the proposal, according to British sources.

An Argentine Foreign Ministry spokesman said in Buenos Aires that Argentina had agreed to a truce in a note from Foreign Minister Nicanor Costa Mendez submitted to the Peruvian ambassador there late Sunday. The note said that for the truce to take effect, Britain would have to suspend its threat to subject all Argentine ships within 200 miles of the Falklands to attack, the official said.

British government sources had said Haig would take back to Buenos Aires "British ideas" for resolving numerous points of disagreement, apparently over conditions Argentina has attached to any withdrawal of its troops from the Falklands, which it seized 10 days ago.

"A number of ideas presented by Haig were agreed upon," said one source, "and will be taken back to the Argentinians as agreed points."

"Naturally we all hope he will be successful," British Foreign Secretary Francis Pym had said of Haig's further talks in Buenos Aires. "There is no doubt that difficulties still remain. We are both still very anxious to resolve this by peaceful means, and we are very grateful to Mr. Haig for all the trouble he has taken."

Thatcher, who saw Haig off from the front door of 10 Downing Street, made no statement.

Haig conferred with Costa Mendez by telephone at least once during yesterday's talks here, which lasted five hours longer than expected. The participants also broke up into smaller groups at times to tackle specific problems. British and U.S. officials refused to discuss details of the negotiations because, Haig said, it would "only complicate the process."

Britain, however, apparently has refused to agree to some conditions Argentina attached in Saturday's talks with Haig to a military withdrawal from the Falklands.

If Argentina agreed to withdraw all its forces from the islands, Britain appears willing to turn around the large naval task force it has dispatched to the South Atlantic and to rescind its threat, which took effect yesterday, to attack Argentine warships within 200 miles of the islands.

But Thatcher's government is not believed likely to agree to reported proposals by Galtieri's government that Argentina leave behind on the Falklands some form or symbol of Argentine sovereignty, whether just the Argentine flag or a civil administration of the islands shared with their 1,800 residents.

Only after a complete withdrawal of Argentine occupation forces and administrators, British government sources again stressed, can the islands' future sovereignty and administration be negotiated. "No solution is then excluded," said one source, "provided the wishes of the Falkland Islanders remain paramount."

British officials refused to say whether an international peace-keeping force or transitional administration following an Argentine withdrawal had been suggested or would be acceptable to Thatcher's government. Pym has said only that "in the course of withdrawal and so forth, it may be that the diplomatic initiative could conceivably contain other elements in it."

Observers said Thatcher cannot be seen here to be allowing Argentina to have gained from invading the Falklands. The political impact of the crisis is still so important that Thatcher agreed yesterday to a request from opposition Labor Party leader Michael Foot that Parliament be recalled Wednesday from its Easter recess to be given a status report by the government.

Diplomatic sources here warned last night against trying to single out specific "sticking points" blocking agreement on an Argentine withdrawal. "We are talking about many points in a very complex situation," one source said.

But they acknowledged that the discussions focused on trying to achieve an Argentine withdrawal and a military disengagement by Britain, rather than long-term questions about the sovereignty and administration of the Falklands.

Since Thursday, Haig has spent 27 hours talking with British and Argentine officials in London, Buenos Aires and then London again yesterday. He also has spent nearly 40 hours flying more than 17,000 miles between the two countries.

Traveling with Haig are Thomas Enders, assistant secretary of state for inter-American affairs, and Vernon Walters, a former deputy director of the CIA who serves as an ambassador-at-large specializing in Latin America. Senior diplomats at the U.S. Embassy also participated in the talks here.

U.N. Secretary General Javier Perez de Cuellar cut short a European visit to return to New York yesterday and declared his readiness to step in if Haig failed in his shuttle diplomacy. He is meeting with the British and Argentine ambassadors to the United Nations this week.

Canada announced that it was joining the 10 European Community nations in banning all imports from Argentina in response to its invasion of the Falklands. Canada also suspended export credits and arms sales to Argentina as requested by Britain.