NEW YORK, JULY 9 -- In an important but not conclusive development in the effort by UAL Corp.'s unions to buy the airline company, five big American banks were reported today to be interested in providing $500 million each in financing for the proposed $4.4 billion buyout.

If the deal goes through, it would make UAL, the parent company of United Airlines, the largest employee-owned company in the nation. UAL's present shareholders would receive $201 per common share, according to the terms announced April 6.

The unions have agreed with the company's directors to come up with ''commitments'' or ''reasonable assurances'' of the financing by Aug. 9. If they are successful, the buyout group, called the United Employee Acquisition Group, will have another four months to close the deal.

The interest by the five banks was confirmed by Steven Matthews, a spokesman for the union group.

''I can confirm that the banks have expressed a willingness to work with our group,'' Matthews said. ''I can confirm that the figure discussed with them is $500 million apiece and that the discussions have revolved around making them {the banks} co-lead managers of the deal.''

He said that the banks may bring others throughout the world into a giant syndicate that would provide most of the financing needed.

The banks that have expressed interest are Citicorp, Chase Manhattan Corp., Bankers Trust Co., Manufacturers Hanover Corp. and Chemical Bank Corp., all in New York.

UAL stock surged on the New York Stock Exchange today, closing at $157.12 1/2 a share, up $4.37 1/2.

Despite the new development, financing the deal remains difficult.

''This is not a done deal by any means,'' said one person closely connected with the Air Line Pilots Association. ''It is just one brick in the wall.'' ALPA is working with United's machinists and flight attendants unions in the buyout effort.

Some analysts were even more pessimistic. It was only last October, they said, that a proposed $6.8 billion management-pilot buyout offer for UAL fell apart when the buyout group failed to find financing.

''This the bare minimum in terms of progress that I would have expected by this time. They only have a month to go,'' said Edward J. Starkman, airline analyst with New York's Paine Webber brokerage company. ''There is nothing now that makes me any more confident or less confident that a deal will be done.''

One problem, some sources said, is that banks might not be willing to provide financing if the transaction does not have at least one outside equity investor. The buyout group is talking to a number of United's airliner and engine suppliers to encourage them to take such an equity position in the reconstituted company.