In defiance of President Reagan's ban on the supply of equipment for the Siberian gas pipeline, a Soviet freighter was loaded today with the first of six British-made turbines that contain American parts. Officials said the ship would sail in a matter of days.

In the meantime, diplomatic efforts were stepped up here to limit the extent of punitive action by the United States against the manufacturer of the equipment, John Brown Engineering Ltd. Reports tonight that the sanctions would be limited only to the gas and oil aspects of the company's operations came too late for a detailed reaction by Brown officials, but any such measures would make completion of the contract substantially more difficult, affecting hundreds of jobs at Brown's already hard-hit Glasgow works.

In addition to numerous official messages on the overall problem, sources said that Special U.S. Trade Representative William Brock would be meeting an as yet undesignated senior British official when he visits London later this week on a previously scheduled private trip.

For the moment, however, sources said there was no compromise formula acceptable to Britain under consideration. Brock suggested over the weekend that the administration might consider substitutes for the pipeline equipment ban if the Europeans put forth alternative measures to increase economic pressure on the Soviets.

The British had taken it for granted that John Brown would be subjected to some American penalties for shipping the turbines, just as two French firms were last week placed on a "blacklist" by the U.S. Commerce Department when they fulfilled a similar Soviet contract. Peter Rees, Britain's trade minister, said today that should sanctions be imposed, the government would be unable to provide any financial aid.

Officials at John Brown have said that the prospect of any U.S. penalties against the firm combined with difficulties in obtaining American rotors for its turbine projects could lead to the loss of as many as 1,700 jobs in Glasgow. That figure is plainly designed to stress the extent of the danger to a region where unemployment is already over 15 percent. The firm has been unsuccessful in finding alternative sources of supply for the rotors that have been supplied by General Electric.

Information about John Brown's activities and financial strength has already been compiled by the Commerce Department so a judgment on sanctions can be made in Washington, sources said. Like much of British industry, the company has suffered in recent years from declining markets and the pipeline deal, worth about $200 million, was seen, the London Times said yesterday, "as vital to its survival" in the Glasgow area.

The 400-ton Soviet freighter Stakhanovets Yermolenko reached Glasgow over the weekend. It will pick up 500 crates before shipping out. The remaining 15 turbines in the order have not yet been manufactured because of the difficulties John Brown has encountered in getting essential American parts.

The problems already faced by John Brown and those it anticipates as a result of American retaliation illustrate why the British are so determined to press ahead with the deal despite the damage it is causing to relations with the United States. Britain's reliability as a trading partner is essential to maintaining badly needed jobs in a serious recession.

But even if Britain were to agree to some American compromise, it could not do so unless West Germany, Italy and France were also challenging the United States. Their positions are, if anything, tougher than the British one, according to diplomats here and at the European Community in Brussels.

The Italians disclosed today that the Soviet freighter Dubrovnik would reach Livorno in several days to start loading turbines made by the Italian conglomerate ENI. That would leave only West Germany to make the first shipments in defiance of the ban.

The Thatcher government's refusal to go along with the U.S. embargo has been applauded by British politicians and the press. Denis Healey, deputy leader of the opposition Labor Party, said today that if U.S. sanctions are imposed on any British firm, similiar actions should be taken against U.S. firms here.

"This is the sort of issue on which the European countries should stand firm and show this very muddled Reagan administration that there are certain things we stand for and clear things we won't put up with," he said.