Vietnamese and Cambodian rebel forces pushed closer to Phnom Penh and Cambodia's only seaport yesterday in their escalating offensive apparently aimed at toppling the Cambodian government, according to Hanoi and Phnom Penh broadcasts monitored here.

In reports broadcast over radio Hanoi, the United National Front for National Salvation, the official name of the Vietnamese-backed Cambodian rebels, made claims of capturing enough new towns to put them in control of almost one-fourth of Cambodia.

From Phnom Penh, the official Cambodian government news agency reported that Neak Luong, a river port 35 miles south of the capital, was strafed and bombed by Vietnamese jets. The Cambodians said a hospital was hit and several patients killed.

Cambodian Premier Pol Pot said today that his troops were battling Vietnamese forces on five main fronts, trying in the northeastern region to cut off invading Vietnamese units from each other. In the southwest they were surrounding the Vietnamese and "destroying them section by section," he said.

The Cambodian leader, making his first public statement since the border war flared into major fighting 10 days ago, said Vietnam "and its boss, Russia, and the Warsaw Pact" had made deep incursions into Cambodia, Reuter reported, quoting Radio Phnom Penh.

The U.N. Security Council was summoned to a closed consultative session today to study the Cambodian complaint against alleged Vietnamese aggression, a U.N. spokesman said. On Wednesday, Cambodian Foreign Minister Ieng Sary asked for an emergency Security Council meeting next week and said a Cambodian delegation would fly to New York to attend.

U.N. Secretary General Kurt Waldheim asked "all concerned to exercise restraint and to refrain from moves that may further escalate the fighting."

In Geneva, the Internaional Red Cross offered to mediate the dispute between Cambodia and the Vietnamese-backed rebels.

The Phnom Penh radio also broadcast what it said was a letter from Prince Norodom Sihanouk, the former Cambodian leader deposed in 1970 by Lon Nol. Sihanouk then became nominal head of the rebel Communist movement against Lon Nol, from exile in Peking, and retired after the Communist Victory.

In his letter, Sihanouk praised the Cambodian army for its valor and asked for support from Southeast Asian and Third World nations.

While the fighting remains concentrated in the northeast, with Cambodia claiming that Vietnamese jets are bombing the privincial capital of Kompong Cham, the greatest escalation apparently is taking place in the south, according to radio broadcasts and official analysts here.

In telligence sources say that one division of Vietnamese troops was airlifted to the Mekong Delta region of Vietnam and these forces are now pushing toward Route 3, a major highway southwest of Phnom Penh that connects it with Kompong Som, the only seaport.

Phnom Penh broadcast that eight fronts have been opened by the Vietnamese and the Cambodian-rebel front. The government also said 14,000 Vietnamese troops were killed in the battles, a doubtful assertion.

Absent from the broadcast was any allegation that the Cambodian troops had pushed the Vietnamese back across the border, a common claim during the previous battles.

For the past several days the Chinese -- Cambodia's sole ally in this war -- have condemned Vietnam repeatedly for its offensive against Cambodia. They have refused to say if they would take any action other than to make proclamations.

The Vietnamese and their Cambodian reble front have captured the provincial capitals of Stung Treng, Kratie and Lomphat in the last week of attacks with armored tanks, heavy artillery, jets and regular forces of about 100,000 soldiers. The rebels, according to Western estimates, number some 20,000.

The current fighting, the most serious in the year-old border war between the former allies, began on Christmas Day. Hanoi has acknowledged no role in the offensive, allowing the month-old Vietnamese-backed rebel front to claim all the victories even though dozens of daily strafing and bombing sorties are being carried out by sophisticated American-and Soviet-made jets.

Depite this offensive, which the Cambodian government describes as a matter of life an death, signs of normality persist in that country. New tourist flights have continued from Bangkok, Thailand, to Angkor, in the untouched northwestern section of Cambodia.

An American congressional delegation plans to visit Hanoi Jan. 10-11, and Waldheim has not abandoned his plan to visit both Cambodia and Vietnam later this month.