Secretary of State Alexander M. Haig Jr. left for Washington tonight to consult with President Reagan after a tense and confusing day that at one point saw his efforts to avert open military confrontation between Britain and Argentina come close to collapse.

He is expected to continue his shuttle diplomacy with a new trip to Buenos Aires after the Washington stopover, but no date was announced for the next round of talks.

Well-informed sources here said the U.S. mission hit a low point early in the day when the Argentines departed from earlier understandings with Haig and set new conditions for a military withdrawal from the Falkland Islands completely unacceptable to Britain.

After a day of telephone calls to Argentine officials in Buenos Aires and meetings with British government leaders here, Haig obtained new proposals from Argentina that he and the British believed could at least allow negotiations to continue, the sources said.

Haig, who arrived at Andrews Air Force Base at 7:30 p.m. Tuesday, said that the parties in the dispute have "received some new ideas which they are considering" and that in the meantime he would consult with the president, probably sometime Wednesday, Washington Post staff writer John M. Goshko reported.

Asked when he might go back to Buenos Aires, Haig said, "It's too early to say. It will be done when we've had a chance to look at these ideas."

He would not say whether he was optimistic or pessimistic about the negotiations.

The British government, in an official statement, said, "Easy optimism would be out of place tonight." Haig called the situation "dangerous and increasingly so, and therefore there is a great urgency in finding a political solution."

But there will be a pause in his shuttle diplomacy, which entered its sixth day today, while Haig consults with Reagan on his next move in the Argentine capital. The biggest obstacle to securing a withdrawal of Argentine troops from the Falklands, in exchange for a British pullback of its naval task force heading for the South Atlantic, appears to be major disagreement between Britain and Argentina about what should be done with the Falklands after an Argentine withdrawal and before negotiations to determine their long-term future.

The crisis began April 2 when Argentine troops, in a move that generated a nationalistic uproar in both countries, occupied the disputed islands in the South Atlantic, sustaining some casualties. The U.N. Security Council called the next day for Argentine withdrawal and on April 5 Britain dispatched a naval task force that is expected to reach the Falklands early next week.

The Argentines, meanwhile, have sent reinforcements to the islands, inhabited by 1,800 British-descended sheepherders, and have vowed to defend them.

Both governments have indicated their desire for a negotiated resolution of the crisis, which provided the spur for Haig's shuttle mission.

The military government of President Leopoldo Galtieri in Buenos Aires wants some immediate form of recognition of Argentina's claim to sovereignty over the islands, while Prime Minister Margaret Thatcher's government here wants some restoration of British administration before further negotiations. They have been unable to agree on compromise proposals for shared or international control during the interim period.

Despite increasing tension and diplomatic sniping over the impasse in negotiations, neither side appears to expect imminent military conflict while Haig pauses in Washington. But the British task force continues toward the Falklands, and the British threat to sink Argentine warships within 200 miles of the islands remains in force, while Argentina's naval commander warned that his fleet was ready to sail "at a moment's notice" to engage the British.

Haig had planned to go directly from here to Buenos Aires after 11 hours of talks with Thatcher yesterday until he was told in late-night telephone calls to Buenos Aires of unexpected changes in Argentina's negotiating position. British sources described these "new formulations" for arrangements to encourage an Argentine military withdrawal from the Falklands as "outrageous" and "unacceptable" to Thatcher.

Haig discussed the problem with Thatcher and her foreign secretary, Francis Pym, during a 90-minute meeting this morning at 10 Downing Street, which raised concern here that Haig's mediation mission might come to an end. A statement released later by the British Foreign Office said, "The difficulties raised by the Argentine government last night created a new and serious situation."

But after telephone calls to Buenos Aires during the day, Haig came back with what British sources called "less outrageous" Argentine proposals. Before leaving his London hotel for Heathrow Airport this afternoon, Haig discussed these with Pym, who described his 70-minute conversation with Haig as "useful."

At the airport before boarding his U.S. Air Force plane for Washington, Haig refused to discuss what he called the "new ideas."

Thatcher's government, according to the Foreign Office statement, "shall remain in touch with the Americans about the new ideas to which Mr. Haig referred . . . on his departure. We hope they may yet provide a way forward. But easy optimism would be out of place tonight."

Diplomatic sources here said there appeared to be divisions in Galtieri's military government over how to proceed in the negotiations with Haig, which have produced unpredictable changes in details of Argentina's negotiating position. The sources said this has added to difficulties that would be expected in complex negotiations involving compromises on emotional issues with national pride and popular support at stake for both governments.

But both Argentina and Britain are reluctant to be seen as the cause of a complete breakdown of the negotiations, the sources said, which was why Haig was able to convince the Argentine government to pull back from the brink today.

The dilemma facing Haig is satisfying both Galtieri's need to preserve some tangible gain from the domestically popular seizure of the Falklands and Thatcher's need to show that Britain, in Pym's words, "does not appease dictators."

On the crucial issue of interim control of the Falklands after a military withdrawal, diplomatic sources refused to say whether Haig had suggested a temporary, tripartite Argentine, British and U.S. administration, which sources in Buenos Aires claim Argentina has rejected. The inclusion of either the Argentine or British flags in such a "three-flag" arrangement would be very difficult for the other of the two adversary governments to justify politically.

Thatcher's position will be put to another political test Wednesday when Parliament, recalled for the day from its Easter recess, again debates the Falklands crisis. Thatcher will open and Pym will end the debate with reports on the government's diplomatic efforts and military preparations to force an Argentine withdrawal.

Four more civlian ships--fishing trawlers to be converted into temporary mine sweepers--were requisitioned by the British government today for its South Atlantic task force. Eleven civilian vessels--two cruise liners, a container ship, three oil tankers and a supply ship in addition to the four trawlers--have now been requisitioned to join the nearly 30 Royal Navy ships in the fleet.