Twelve Vietnamese divisions - close to 100,000 troops - have taken up positions along the Cambodian border as tension has risen between the two Communist neighbors, U.S. officials reported yesterday.

A war of words between Hanoi and Phnom Penh has stepped up sharply, most dramatically in an open press conference and the release of self-justifying documents yesterday morning in the Vietnamese capital. Indications are growing, according to officials monitoring the situation here, that a wider war of bullets and bombs may not be far off.

"It is Cambodia which has caused the tension in the Vietnam-Cambodia border situation, deterioration of the Vietnam-Cambodian relationship and great difficulty and implications for all efforts aimed at settling the questions of improving relations between the two countries," charge Radio Hanoi's Cambodian-language account of the "solemn press conference" of foreign ministry offical Nyo Dien at the Hanoi International Club.

One of two lengthy documents handed out at the press conference, according to the radio account, is a description of "Cambodia's provocations, enroachments and large-scale and systematic military attacks on Vietnamese territory during the past three years."

Earlier this week, in a broadcast strikingly reminiscent of those to Saigon enemy troops in earlier years, Hanoi appealed to "beloved Cambodian soldiers" to rebel against "the present powerholders" in Phnom Penh. "In your ranks, many are turning their guns around," said the broadcast invitation to mutiny.

Cambodia for its part has recently voiced radio attacks on "the Vietnamese enemy's ambition, agreesiveness, trickery, ferocity, fascism, and savagery." Phnom Penh repeatedly charges that the Vietnamese are seeking to destroy the Cambodian revolution had "make Cambodian their satellite in an Indochinese federation." The Cambodian Communist Party has also launched a public campaign to "strengthen party discipline" against the threat from across the border.

U.S. intelligence has some reports that Vietnam is training groups of Cambodians, evidently to undertake political or military missions in their homeland. Open attacks during the past three weeks against Cambodian Premier Pol Pot, whose regime was described by Radio Hanoi as "a hell on earth," have led officials here to conclude that Vietnam is now aiming to overthrow the Phnom Penh regime.

The bitter falling out of the two neighbors, reaching this new peak three years after the red flags were raised over Phnom Penh and Saigon, is believed to be based on age-old hostility between the two peoples as well as the radically different nature of the two victorious regimes.

The Vietnamese moved with caution and deliberateness against the economic and social fabric of the former South Vietnam and sought trade and diplomatic contact with the world outside. The Cambodians, on the other hand, moved with fierce determination to stamp out all vestiges of the old regime, ousted or confined all diplomats except those from China, and have shut up their country form the outside world.

Skrmishes between the two regimes began shortly after their respective victories in April 1975. The trouble became serious when Vietnam launched several thousand troops into Cambodia's border area to stop shelling and harassment a year ago this month.

Heavy fighting erupted in a series of battles and shelling engagements with Vietnam waging a conventional struggle using heavy weapons and Cambodian fighting back with surprising effectiveness using hit-and-run and guerrilla tactics.

Vietnam issued a call for a ceasefire, mutual withdrawal and negotiated settlement on Dec. 31 and again Feb. 5. This was repeated in Hanoi yesterday. A high-level Chinese delegation headed by Madame Ten Yingchao, widow of Chinese Premier Chou En-lai, flew to Phnom Penh in mid-January in an unsuccessful effort to mediate.

Some U.S. intelligence reports suggest that Chinese ships landing in Cambodia early in February delivered new military equipment, including the 130-mm, artillery Cambodia has been using recently.

The much-augmented Vietnamese force near the Cambodian border - up from about three divisions normally stationed in that area - in believed to be armed in part with captured American armored personnel carries, F5 jet fighters, Huey helicopter gunships, 105-mm artillery and other U.S. material.

Vietnamese Defense Miniter Vo Nguyen Giap and Communist Party General Secretary Le Duan are reported to have visited the border area.

A major Cambodian attack in mid-March drove Vietnamese troops and civilians out of the border town of Ha Tien. Hanoi has been broadcasting interviews with survivors charging grisley atrocities.

In the view of U.S. officials, the attack on Ha Tien pushed Vietnam into a new phase of bitter invective and military preparations. The Vietnamese retook Ha Tien, but retaliation for the bloody episode is yet to come.