The State Department yesterday said the United States "is deeply concerned" about Vietnam's attack across the Cambodian border into Thailand, but cautioned that it is too soon to speculate about whether the U.S. security treaty with Thailand might eventually be invoked.

The expressions of concern came from both Deputy Secretary of State Warren Christopher and department spokesman Hodding Carter in separate meetings with reporters yesterday.

Vietnamese forces moved across the Thai border Monday morning, attacking in the region of border camps housing thousands of Cambodian refugees.

Both U.S. officials yesterday described the situation as "real and serious." But they added that reports about what is really going on were still fragmentary and murky because the action was in remote border regions and therefore the extent and motive for "this further aggression" by Hanoi remained unclear.

Carter said elements of two Vietnamese regiments appeared to be involved in the attack in two areas. Christopher said that the scale of the attack -- based on information available early yesterday -- may still be relatively small, involving hundreds of Hanoi's troops rather than the 2,000 or so mentioned in some news reports.

Carter said casualties among the refugees ran into the hundreds, that tens of thousands of others had fled the camp areas and that the relief programs aimed at feeding many refugees in the western reaches of Cambodia had also been broken off by the attack and fighting.

Carter described Thailand as this country's "close friend and ally, with whom we have a security treaty." The United States he said, "will stand by its commitments" to Thailand, and as a first step he suggested that the United States "will be considering" what more it can do in terms of helping Thailand buy additional weapons.

Washington was in close consultation with Bangkok over the current incursion, Carter said, but both countries were still a long way from invoking the 1954 Manila Pact security treaty, and Washington had not yet been asked by Thailand to invoke it.

Carter pointed out that "we are not even past the first step" in the invocation process, which involves initial consultations between the two countries, then an agreement on the nature of the threat and steps needed to counter it, and then consultations with Congress before acting.

"I don't want to start speculating about anything that may occur," Carter said, "until we are able to resolve what is occurring.

The pact contains two key elements.

One requires the parties to consult if, in the opinion of any party, a threat exists to its territory, sovereignty or independence. The other applies to armed aggression, and in that case each party agrees "to act to meet the common danger in accordance with its constitutional processes."

Australia, New Zealand, the Philippines and England are also pact members, but the United States, in the past, has made it clear in bilateral discussions with Bangkok that it is committed to the security and territorial integrity of that country.

In a related development, the Pentagon announced yesterday that the Bangkok government will receive 35 modeled U.S. M48 tanks, with deliveries between September and December.

The $23.5 million sale of these tanks -- which are improved models of older versions, with bigger cannons and night vision devices -- had been agreed to long before the Vietnamese attack, and officials said the timing of the announcement had nothing to do with the border clashes.

The Thai army, the Associated Press reported yesterday from the Pentagon, has only about 170 medium and light tanks. Vietnam has 1,400 to 1,500 tanks.