President Reagan and South Korean President Chun Doo Hwan called yesterday for intensified dialogue between North and South Korea to reduce tensions on the divided peninsula, but warned of possible North Korean military adventures.

In a White House departure ceremony after a two-hour meeting and working luncheon, Reagan praised South Korea's "creative approach in engaging North Korea in direct talks," which are expected to resume on economic and humanitarian issues next month.

Reagan also expressed concern, which he said Chun shares, about "the continuing forward deployment of North Korean forces toward the demilitarized zone."

Gen. William J. Livsey, commander of U.S. forces in South Korea, said earlier this month that 80,000 to 100,000 North Korean commandos have massed near the demilitarized zone and may be preparing for some kind of military action.

Reagan said he and Chun had agreed that "this deployment heightens the need for vigilance on our part."

The South Korean president, a former general who took power in a military showdown in 1980, said that "the next few years will be a crucial period for the prevention of another war on the Korean peninsula."

He expressed confidence that U.S. and South Korean cooperation could cope with any "military adventurism or terrorist attacks" from the communist north.

A senior Korean official said Chun's remark about the "crucial period" ahead referred to the fact that South Korea continues to outpace North Korea economically, while the north hangs onto an advantage in military capability.

Another factor introducing new uncertainty is possible North Korean instability as President Kim Il Sung is replaced by his son, Kim Chong Il. A South Korean official said this may take place late this year.

On the South Korean domestic scene, Reagan spoke of his support for Chun's "commitment to a peaceful transfer of power at the end of his term in 1988." Chun often has pledged to give up power after his term in keeping with the constitution, which his predecessor as president, Park Chung Hee, refused to do. Chun's statement here and its reiteration by Reagan appear to give added weight to his commitment.

Reagan and Chun discussed the Korean domestic scene primarily in a small Oval Office meeting, before a larger meeting of the two delegations.

The mens' meeting took about 50 minutes, nearly twice as long as had been planned.

At a White House briefing on the talks, a senior State Department official reported that Chun said some "general things" about his intention to improve the political climate in South Korea leading up to the 1988 elections, but that no details were given.

South Korean sources said that neither in the White House session nor in a subsequent meeting of senior officials at the State Department was any mention made of the turbulent return home two months ago of opposition leader Kim Dae Jung or the status of restrictions on his movements and political activity. Kim was released from house arrest March 6 but some limits on his activities remain.

As Chun conferred with Reagan, more than 100 anti-Chun protesters in Lafayette Park demonstrated against human rights abuses in South Korea.

At a news conference later, Pharis J. Harvey of the North American Coalition for Human Rights in Korea charged that the Chun government has tortured student dissidents and forced them to sign false confesssions.

Reagan, in his remarks, said "considerable progress" has been made in advancing political freedoms in South Korea.

State Department officials pointed to increasing recognition that is now being given to the political opposition there following the Feb. 12 parliamentary elections, in which opposition parties achieved surprising gains.