Vietnamese military forces in western Cambodia have set this weekend as the target date for launching a major offensive against insurgent forces near the Thai border, according to intelligence reports reaching Washington.

Such an operation could spill over into Thailand, a U.S. ally, embroiling the United States in another international confrontation even while the crisis with Iran continues.

State Department officials said last night there is no evidence that Vietnamese forces intend to attack Thailand in their expected drive, nor is there evidence that Vietnam plans deliberate incursions into that country.

However, the officials did not rule out Vietnamese operations across the border in pursuit of Cambodian insurgents, nor a stampede of Cambodians in border area camps into Thailand to escape the fighting.

Rep. Lester Wolff (D-N.Y.), chairman of the House Foreign Affairs subcommittee on Asian and Pacific affairs, said "raids" or "an invasion" by Vietnamese forces into Thailand "would be of the gravest consequences for peace and stability in Asia." In light of information supplied to him yesterday, Wolff called on U.S. policymakers to exert maximum influence through the Soviet Union and other world powers to head off a Vietnamese attack into Thailand.

Thailand is allied to the United States by the 1954 Manila Treaty setting up the South East Asia Treaty Organization (SEATO). A U.S. military commitment to support Thailand against communist aggression was made more specific in a 1962 joint statement.

President Carter, in a speech on defense Dec. 12, declared that the United States will meet its responsibilities to Thailand and other Asian nations. Administration officials said then that the United States will provide military aid if the Vietnamese offensive spills over into Thailand.

The expected Vietnamese sweep in the Thai border would be the next step in Vietnam's takeover of Cambodia, which began in massive fashion with an invasion last Christmas. Nearly 200,000 Vietnamese troops have poured into Cambodia, but they continue to be harassed by Cambodian/guerrillas led by Pol Pot.

The Soviet Union is supplying military and economic support to Vietnam, while the People's Republic of China is aiding Pol Pot's forces. This proxy war of the rival communist great powers threatened to take on worldwide dimensions in February when Chinese forces briefly invaded the northern part of Vietnam to "teach a lesson" to their neighbor.

At that time, the Soviet Union took no military retaliation against China. Thailand was not involved. The United States called on China and Vietnam to stop fighting, but said it would not become involved in a battle between Asian communist states.

With the shift of military action to the western part of Cambodia, the danger has grown that the battle may involve Thailand. The presence of 600,000 Cambodians in makeshift refugee camps straddling the Thai border complicates the task of the Vietnamese and adds to the problems of the Thais.

U.S. intelligence reports suggest that 30,000 to 40,000 Vietnamese troops have been preparing for "dry-season" weeps against 20,000 to 30,000 Pol Pot fighters in western Cambodia. Both sides are well armed.

Among the immediate U.S. concerns in case of a military flare-up are the political stability and territorial integrity of Thailand, the possibility of a renewed Chinese attack on Vietnam to relieve the pressure on Pol Pot, and the threat of Soviet counteraction.

The credibility of U.S. assurances to noncommunist Southeast Asia, Japan and South Korea also would be at stake if Thailand is threatened. But U.S. fleet movements to "show the flag" currently are inhibited by the assignment of many ships to the Persian Gulf area in the continuing conflict with Iran over the American hostages there.