Negotiations between United Airlines and the machinists union to end the six-day-old strike broke down last night almost before they had atarted.

Shortly before 11 p.m., negotiations for both sides checked out of the Mayflower Hotel where each group had met separately with federal mediator Robert O. Harris.

A spokesman for United said he couldn't confirm any breakdown but said the company would have a statement in the morning.

The breakdown apparently came even before the two sides had any face-to-face meetings.

The airline was struck at midnight Friday by the International Association of Machinists and Aerospace Workers after its members voted overwhelmingly to reject a second proposed contrast with the company. A first tentative settlement had been rejected March 15. The agreement rejected last week called for a 31.9 percent average increase in wages over three years, plus cost-of-living adjustment.s

Earlier yesterday, United announced that it was canceling all flights through the Waster weekend because of the strike. The action was taken yesterday to give holiday travelers as much time as possible to seek alternate travel arrangements, an airline spokesman said. Even if a tentative settlement could have been reached quickly - something unlikely now - the agreement would have to be submitted to the rank-and-file membership for ratification before the walkout could end. CAPTION: Picture, United Airlines planes, grounded by strike, crowd a parking apron at Los Angeles International Aiport. AP