The Reagan administration has gained agreements from five steel exporting nations to reduce their sales in this country, but a sixth major supplier -- South Korea -- walked out of talks in a huff and it remained unclear whether negotiators would meet the president's goal of sharply cutting steel imports by today.

While agreements with five nations are in hand, trade negotiators in the office of U.S. Trade Representative William E. Brock were unsure last night how much the cuts would amount to and whether they would bring the level of imports down to 18.5 percent, the figure President Reagan called for 90 days ago.

"We are lower than 26 percent the present level of imports , but how close we are going to get to 18.5 percent I really don't know," said USTR spokesman David Demarest.

The negotiations are the cornerstone of President Reagan's steel program, announced Sept. 18 when he rejected an International Trade Commission recommendation to cut imports through a combination of quotas and increased tariffs. Instead, the president ordered Brock to gain voluntary import restraints from foreign suppliers by today. The presidential aim was to reduce imports to about 18.5 percent of the American market.

Brock's negotiators have used the stick of unilateral presidential action to cut imports and pending unfair trade cases against some suppliers to win agreements.

Japan, the largest exporter of steel to the United States, two weeks ago became the first nation to agree to reduce its sales in this country. Although details still need to be ironed out, Japanese steel makers are likely to cut exports to about 5.8 percent of the American market from its present 6.3 percent level.

Four other suppliers -- Spain, Brazil, South Africa and Australia -- also agreed to cut their steel sales here, and talks with Argentina will continue today although it and the United States are far apart.

Spain, Brazil, South Africa and Australia account for 4.2 percent of the U.S. market. Along with Japan, those four nations account for about one-third of America's steel imports.

Spain's exports tripled from last year to this, jumping from 446,000 tons in the first nine months of 1983 to 1.2 million tons in the first nine months of this year. Brazil's imports also increased, but not as much -- from 892,000 tons to 1 million -- while South Africa sold 493,000 tons and Australia sold 170,000 tons.

South Korea, the fourth-largest importer to the United States behind Japan, the European Community and Canada, stormed out of talks last week with Deputy U.S. Trade Representative Robert Lighthizer, accusing America of favoring Japan over other steel-exporting nations.

Korea offered to keep its steel exports at this year's level of 2.4 percent of the American market, but Lighthizer reportedly wanted to reduce its shipments to its 1983 market share, 1.7 percent. This is a sharper reduction than the agreement reached with Japan.

Demarest called South Korea "the major holdout" and said that during three rounds of steel talks the Koreans had been "the toughest negotiators" of all the nations Lighthizer has dealt with. It remained unclear whether Korea would return to the talks, although there were reports that the administration was putting pressure on Seoul to accept a compromise figure, 1.9 percent of the U.S. market.

The EC already limits its exports to the United States under a 2 1/2-year-old agreement, and Canada's exports are expected to remain at present levels.