South Korea expressed its disappointment and Japan said "it is regrettable" that President Reagan signed a historic trade reform bill aimed at opening overseas markets to American goods and exports.

The signing of the bill Tuesday ushered in the U.S. response to the difficult international markets American products must compete in, and two of America's largest trading partners deplored the action they saw as protectionist."It is regrettable," said Noboru Takeshita, Japan's prime minister. "We hope the U.S. administration will take appropriate measures to curb protectionism."

Japan, which accounted for about one-third of the $171 billion U.S. trade deficit in 1987, was the primary target of the bill's provisions.

Japan's top trade official, Hajime Tamura, said the provisions may "trigger further protectionist pressures, endangering the future development of the world's free trading system."

But Japan was not the only object of the bill, which was also aimed at Korea, Taiwan and Hong Kong, the so-called newly industrialized countries that have greatly benefited from U.S. trade.

In Seoul, the government said it hoped the bill does not set off a flurry of protectionist legislation around the world.

"The government of the Republic of Korea expresses its deep disappointment at the fact that the omnibus trade bill has been finally signed," a statement by Foreign Minister Choi Kwang-soo said.

"We are particularly concerned with the likely counterreactions from U.S. trading partners to protectionist trade policy envisioned in the trade act, which may entail enactment of similar laws or retaliations in kind, thereby creating a confrontational world trading environment," Choi said.

On signing, Reagan expressed some reservation about 2,000-page trade bill. "Let me be plain," Reagan told congressmen and dockworkers gathered in Long Beach, Calif., to witness the signing. "There are some things in this bill I don't like."

The bill, passed overwhelmingly in the House and Senate after three years of debate, is intended to make the United States a leader in world trade by opening foreign markets to U.S. exports and requiring retaliation against foreign trade abuses.