The government's index of leading economic indicators rose a slight 0.1 percent in September, the Commerce Department reported yesterday.

The September increase was the fifth in a row and the ninth in the last 10 months.

According to the Commerce Department, five of the 11 statistics combined in the index rose, five fell and one remained constant.

The index is compiled by economists to forecast developments in the nation's economy.

Secretary of Commerce Malcolm Baldrige said in a statement that "the modest rise in September's leading index gives us a string of five consecutive gains, averaging 0.4 percent per month and pointing to continued economic growth in 1986.

"Key leading indicators directly related to forthcoming production are pointing upward," he said.

The 0.1 percent increase was smaller, however, than the 0.7 percent increase the index registered in July and the revised 0.9 percent jump in August.

Some private analysts said yesterday the pace of economic growth would be sluggish in the first half of next year and pick up somewhat during the final six months of 1986.

White House spokesman Larry Speakes said the current economic recovery "can only be limited by a failure of the Congress to act on its historic opportunity to balance the budget."

House and Senate conferees have been trying to compromise on a plan to balance the federal budget for weeks.

The United States is nearing the end of three consecutive years of economic growth, although economic output and job creation has been far slower in recent months than in 1983 and 1984.

Economic growth nearly always slows later in an expansionary period.

David Wyss, senior vice president of the economic consulting firm Data Resources Inc., said yesterday's report is consistent "with a pattern of continued but sluggish" growth.

He said he anticipated that 1986 would look very similar to 1985 -- with the economy growing faster in the last half of the year than in the first.

The gross national product, the broadest measure of the output of the U.S. economy, rose at an annual rate of 1.1 percent during the first half of 1985 but accelerated to an annual rate of 3.3 percent in the third quarter.

In the first half of next year, consumer buying is expected to slow as households become reluctant to borrow more -- both because they already have a high stock of debt and because low inflation is reducing the size of wage and salary increases.

Wyss predicted that growth would accelerate in the final months of 1986 because, by then, U.S. manufacturers would begin to benefit from the decline in the U.S. dollar that started last spring.

Because the dollar has been so strong, many products manufactured in the United States are far more expensive than competing products made in other countries.

The strong dollar both inhibits the ability of many U.S. companies to sell their products abroad and makes imported products more attractive in the United States.

As a result, the United States has been running record deficits in its trade relationships with other countries.

The Commerce Department reported yesterday that in September the United States imported $15.5 billion more goods than it exported, a sharp increase from the $9.9 billion trade deficit recorded in August.

The September deficit was the biggest monthly imbalance in history and 80 percent of it was because of heavy imports of manufactured products.

The Commerce Department said that the five measures in the leading indicators index that rose in September were the size of the money supply, contracts for capital equipment, the length of the average work week, building permits and a rise in the prices of materials that are sensitive to a pickup in demand.

They were offset by a decline in stock prices, the level of credit, net business formations and new orders for consumer goods.

New claims for unemployment benefits rose. An increase in claims is considered a negative development for the index. Another indicator, which measures the backlog in deliveries by factories, did not change.

The department also reported that orders to U.S. factories for manufactured goods fell 0.6 percent in September, mainly because of a steep 17.1 percent drop in orders for defense equipment.

Orders for defense equipment are volatile. Had they been factored out, factory orders would have risen 0.3 percent in September, the Commerce Department said.