In barely two weeks the simmering border war between Vietnam and Cambodia erupted into an offensive that now threatens the existence of the Cambodian government.

Analysts here and in Asian capitals are calculating when the Vietnamese and their Cambodian rebel allies will encircle Phnom Penh and force the Communist leadership to abandon the capital.

Yesterday, Cambodian Prime Minister Pol Pot made his first radio address since the Christmas offensive began, extolling the people not to surrender but to join him in a "pledge to fight them to the end to keep our national prestige."

"Will our people and revolutionary forces let them engulf out land, eliminate the Khmer people?" Pol Pot asked, answering his own question with a fiery "no."

But the battle reports monitored here suggest the opposite. The Vietnamese Army and Cambodian rebels are moving swiftly across the south, southeast and northeast with what analysts say is the aim of isolating Phnom Penh from its supply routes and contact with its army units.

The Cambodian government, which for the last three years has been one of the most isolated in the world, suddenly is appealing to the entire international community for help, even to the United States, which has condemned it as one of the worst violators of human rights.

Each day this week, broadcasts by the month-old Cambodian National United Front for National Salvation, the official name of the Vietnamesebacked rebels, has claimed victories that have proved "deadly accurate," according to American government specialists making their own analyses.

Yesterday, Svay Rieng, a key city in the Parrot's Beak that juts into Vietnam, became the latest provincial capital claimed by the rebels in their broadcasts.

New information also suggests that although the Cambodian army is fighting fiercely, the farmers in the countryside are not resisting the advancing Vietnamese and rebel troops.

Another indication of swift deterioration came in the announcement by Pol Pot that his army is switching to guerrilla tactics rather than continuing to face the Vietnamese and rebels as a regular army.

The major fighting reported yesterday was on four fronts, involving many towns well known during the 1970 U.S.-South Vietnamese offensive in the same area.

Along the Gulf of Siam, the Vietnamese were marching on route 3 past the city of Kampot. In the southwest, Svay Rieng was captured and the Vietnamese and rebel forces were moving along Route 1 west toward Phnom Penh. Route 7, from the border to the Mekong River, was reported secured by the anti-government forces.

There were scattered reports as well of another advance up Route 2 toward Takeo.

For the past several days the Cambodians have looked to the United Nationas for help and yesterday the U.N. Security Council met in a closed session at Cambodia's request.

Cambodia is unlikely to find aid from that body, however, since the Soviet Union, Vietnam's main ally, has veto power there.

The Vietnamese delegation at the United Nationas issued a statement accusing the Cambodian government of "slanderous allegations" and "flagrant calumnies" that "can in no way cover up [Cambodia's] heavy defeats in the face of the heroic Kampuchea people's just struggle for national salvation."

The rebel front joined in denouncing the scheduled U.N. Security Council debate on the war.

Last month, officials in Cambodia were obsessed with what they called "the Vietnamese threat to swallow Kampuchea" but at the same time they said they were confident that their troops could defeat any new drive along the border.

The strength of the offensive obviously caught them by surprise.

In the first major border clash last January, the Cambodians did drive the Vietnamese back and invaded Vietnamese provinces, leaving corpses of beheaded and disemboweled Vietnamese peasants. Visiting European journalists photographed these bodies while the Vietnamese expressed contempt for the Cambodian Army, calling the soldiers "savages."

As late as Dec. 22, the Cambodian officials in Phnom Penh still showed confidence. The Vietnamese "are fighting with half the strength of last year," one military officials said. "They fight with planes and tanks; we fight with infantry and that is where the strength lies."

Early Saturday morning Cambodia claimed major victories. Radio Phnom Penh said its forces killed nearly 1,000 invaders and destroyed 69 tanks and four trucks in heavy fighting over the past three days, Associated Press reported.

[Cambodian insurgents, however, claimed to have captured another provincial capital. Takeo, 37 miles south of Phnom Penh, thus taking control of much of eastern and northeastern Cambodia.]