hai Prime Minister Prem Tinsulanond today pledged to stiffen resistance to incursions from Cambodia by Vietnamese troops as the United States delivered the first shipment of urgently requested weapons to the Thai armed forces.

Prem, a retired Army general who is touring the volatile Thai-Cambodian border area, made the statement after inspecting an array of captured Vietnamese military equipment at this regimental headquarters.

After the tour, Prem and the Army commander-in-chief, Gen. Arthit Kamlang-ek, flew to Surin Province, where a Thai Air Force A37 attacker-bomber crashed yesterday in mysterious circumstances.

Arthit refused to confirm or deny press reports that the plane was shot down by Vietnamese ground fire. Senior military officials said the crash was still being investigated.

The plane was patrolling near Cambodia's northern border with Thailand when it crashed less than a mile inside Thai territory, the Air Force said. The pilot and copilot were killed. Thai fighter-bombers have staged two air strikes against Vietnamese troops who are alleged to have crossed into Thailand during their current offensive against Cambodian resistance groups along the border. But the border area has been relatively quiet for the past several days.

"This is not the first time that Vietnamese troops have invaded Thai territory," Prem said at this military base about 20 miles north of the Thai border town of Aranyaprathet. "But this time it is more serious than the other times. This time it seems like the Vietnamese troops have the intention to invade."

As Prem and Arthit were touring the border, a U.S. military transport plane arrived in Bangkok with a shipment of Redeye ground-to-air missiles purchased by Thailand late last year. The U.S. Embassy in Bangkok said the shipment was in response to a Thai request for accelerated arms deliveries in view of the fighting along the border.

The embassy said additional shipments during the next 10 days would include the new M198 model 155mm howitzer plus ammunition and propellant. The long-range artillery pieces are being drawn from the U.S. Army's own inventory to underscore American support for Thailand, the embassy said.

The guns, with a range of 16 miles, are intended to counter the new Soviet-supplied 130mm artillery being used by Vietnam.

Thailand claims to have bettered the Vietnamese in several battles since the beginning of the month. According to an Army statement issued today, more than 250 Vietnamese soldiers were killed and many others were wounded. Thailand said five of its troops were killed and 20 wounded.

An aide to Arthit said that while Vietnamese patrols continued to cross into Thailand, there were no longer any troops dug in on Thai territory.

As proof of the incursions, Thai authorities displayed captured Vietnamese equipment including machine guns and ammunition, pith helmets, gas masks, a recoilless rifle and a section of tank tread. They said all of it was recovered from Thai soil, some as recently as April 6.

Prem declared that "this display of weapons shows the determination of the Thai armed forces to defend our territory."

In Phnom Penh, meanwhile, the foreign minister of the Vietnamese-installed Cambodian government, Hun Sen, said that the fighting at the border involved regular operations by his government's forces to drive out communist Khmer Rouge guerrillas "and other Khmer traitors armed by reactionary circles." He blamed the United States, China and Thailand for the fighting.

Thai officials and western diplomats have said, however, that no troops of the Phnom Penh government's army have been involved in the current offensive.