The sudden resignation of South Korea's figurehead president paves the way for the early succession of Gen. Chon Doo Hwan -- possibly within the next two weeks.

Government sources said today that a new president will be chosen promptly and barring an unexpected switch in plans, it will be Chon, the ambitious general who has rapidly climbed to the top since helping to organize a coup within the military last December.

There was no clear explanation why Chon and his coterie of fellow generals decided to take power formally much earlier than expected.

President Choi Kyu Hah, who resigned this morning, had been expected to continue in office until next year when, according to a generally accepted timetable, a new president would be elected under a new constitution.

The United States, in a brief statement issued by the State Department today expressed "deep concern" about the resignation of Choi. The statement said the concern arises from the U.S. belief that "movement toward a broadly based civilian government is essential to the stability of the Republic of Korea."

The U.S. statement, which did not mention Chon, said the selection of Korea's leaders and nature of its future constitution are "matters to be resolved by the Korean people."

South Korean sources said the electoral timetable is to be considerably accelerated. It now calls for a successor to be named by about Aug. 25 by the National Conference on Unification, a sort of electoral college.

A new constitution is to be promulgated quickly and submitted to a referendum in September or October. That would be followed by the selection of a president by a new electoral college.It is believed that Chon will be chosen both as the interim president and then named the first chief of state under the new constitution.

None of those developments would alter the real power situation in South Korea, which has been run by Chon and his associates since they seized total control in May following massive student demonstrations. They have ruled under increasingly rigid decrees through a structure known as the Special Committee for National Security Measures.

Chon at one time had said he is planned to return to his military duties when a state of emergency had passed after the assassination of president Park Chung Hee last October. But in recent days he had hinted that he might leave the military and seek the presidency. There has been a widespread campaign in the press, which has military command censors, to build support for him as a civilian leader and for the drastic purges of government and political leaders engineered by him and his colleagues.

For several months it was believed that the military would anoint some acceptable civilian as its presidential candidate to preserve the impression of civilian rule, but that plan apparently was abandoned in favor of placing Chon, minus his uniform, in the presidency.

With the government firmly under control and opposition ranks decimated by arrests and purges, the military group should have no trouble elevating its man. The principal opposition leader, Kim Dae Jung, is on trial for his life and another, Kim Young Sam, renounced politics after being under house arrest for months.

Before the final military crackdown in May, both political parties had been planning the direct election of a president under a revised constitution. The plan now, however, calls for creation of the new 5,000-member electoral college to choose a president for a single seven-year term, one year longer than the terms served under the existing constitution. Both the electoral college and members of a new National Assembly would be chosen in a national election. The present National Assembly has been forbidden from holding meetings.

choi had been Park's prime minister and was elevated to the presidency as an interim leader after Park was shot by an assassin Oct. 26. Although he had been close to 'park for years, he presided over a period of unprecedented political reform during which dictatorial decrees were abolished, political prisoners were released, and workers were permitted to strike.

After the military takeover in May, Choi was reduced in power and the reins of government were grabbed by Chon and the military-dominated Special Committee. The reforms were abandoned and thousands have been arrested or purged from jobs and offices.

In his resignation statement this morning, Choi said he was leaving office to permit a peaceful transfer of power and to hasten the advent of a new period of stability in South Korea.

"My purpose is to provide a historic turning point for the building of a new, happy community of stability, morality and prosperity, which is the demand of our times," Choi said. "I intend to put an end to our unhappy political history by establishing the precedent of peaceful transfer of power and to eliminte the climate of mutual distrust by practicing politics of responsibilty."

He also took partial blame for the period of unrest last spring, which culminated in large student demonstations and a bloody uprising in the provincial city of Kwangju. Choi said he has been "painfully aware of my own political responsibilities for having caused great anxiety among our people" because of those disturbances.

Choi's temporary successor is Park Chung Hoon, a businessman who has served briefly as acting prime minister. In a statement today, he said the country's most pressing need is to "shorten the period of transitional government and forestall any confusion or paralysis of state functions resulting from a vacuum of national leadership.