Secretary of State Alexander M. Haig Jr. charged today that Soviet "threats and threatening letters" have created a seriously worsening situation in Poland, and warned against "any external or internal repression" generated by Moscow.

Haig, on the first stop of a 16-day Asian and Pacific tour, called an unexpected news conference to highlight the latest episode of growing U.S. concern about Poland. While dealing with tensions in Europe, he also addressed new troubles for American diplomacy both in the Middle East and China.

The secretary of state arrived here from Washington early this morning after a journey of nearly 22 hours. The two-day visit to this teeming British crown colony is primarily a rest stop on the way to the Reagan administration's first meeting with Chinese leaders, scheduled to start Sunday in Peking.

Haig's journey to China has become more complicated in the past several days by renewed and repeated denunciation from Peking of U.S. arms sales to Taiwan. Seeking to cool the rhetoric, Haig said today his mission is "not an arms-selling operation for either Peking or Taiwan," and he declined to repeat publicly the U.S. stand that its military sales to Taiwan will continue.

Haig said his principal aim in Peking is to "clear the air" with respect to the administration's policies on China and discuss a "special role," which he did not specify, for Sino-American relations in Asia. He will discuss potential U.S. arms sales to PEKING IF chinese leaders ask, he added.

But Poland more than Peking was clearly at the top of Haig's agenda in the unusual on-the-record news session, which was evidently intended to lay down a new U.S. marker of concern and warning amid a "seriously deteriorating" situation in Warsaw.

Unlike the earlier high points of anxiety in December and late March, Haig laid stress on political factors rather than any new signs of Soviet military preparations.

The secretary of state singled out for criticism the recent letter from the Soviet Politburo to the Polish Communist Party, which he termed "very threatening," as well as "an increase of Soviet threats" through propaganda.

Haig spoke of the internal Polish situation as being more tense and "the threatening posture of the Soviet Union" as "very worrisome."

Under questioning, he declined to explain or expand his warning against "internal suppression" in Poland, other than to say that the Soviets have "a number of options open short of direct external intervention."

Asked if he was asking the Polish leadership not to crack down on the country's liberalization, Haig replied: "We would not presume to intervene in the internal affairs of Poland. We don't expect others to do so."

Any Soviet intervention in Poland, Haig said, would have "major impact" on U.S.-Soviet negotiations of all sorts. He said preliminary talks began this week, in Washington, on limitation of theater nuclear weapons in Europe. t