Only one major city remains in the hands of the Pol Pot government in Cambodia and effective, organized resistance to the Vietnamese-led insurgent forces could be over within a week, according to intelligence analyses in Bangkok and Washington.

Vietnamese forces, including heavy armor, have laid siege to Battambang, the country's second-largest city after the capital, Phnom Penh, and are pushing close to the Thai border, according to news agency reports from Bangkok.

"Resistance continues around the country, but we seriously doubt that Pol Pot or Ieng Sary will be able to claim in a week or so that they have an effective army, a claim that they are making today," one intelligence official said here yesterday.

Ieng Sary, foreign minister of the government ousted from Phnom Penh this past Sunday, arrived in Peking yesterday after escaping from the country through Thailand. He was greeted by Chinese Vice Premier Keng Piao, according to news services, but there were no indications of his plans. The Chinese have been virtually the only supporters of the Pol Pot government.

In contrast to previous reports, a U.S. official said yesterday there is now reason to believe that Pol Pot, the head of the ousted government, may be alive.

"We believe Pol Pot is out there," he said. Chinese, Yugoslavs and others who escaped the invading forces have reported in Thailand that many leaders of the government managed to slip out of Phnom Penh before the invading forces arrived and that Pol Pot may well have been among them, the official said.

Pol Pot vowed in the early stages of the Vietnamese-led attack, which began Dec. 25, that he would move into the countryside for a campaign of guerrilla warfare if necessary. Until now, however, most reports assumed Pol Pot had been killed, while his chief aides, Ieng Sary and Khieu Samphan, had escaped.

Until Ieng Sary contacted the Thai government a couple of days ago, nothing had been heard from any of three. Khieu Samphan reportedly was seen in the border region from which Ieng Sary left for Thailand. There is as yet no firm proof that Pol Pot is alive, but analysts apparently no longer take this as an indication he is dead.

Whether the leaders remaining in the country will be able to organize an effective opposition to the Vietnamese and the new government in Phnom Penh remains another question.

All sections except the western areas adjacent to the Thai border now appear to be under Vietnamese control, according to the reports here and in Bangkok, although isolated pockets of resistance remain in all regions.

The Vietnamese force that swept through Siem Reap, 110 miles from the Thai border, reportedly moved on in a tank-led column to Sisophon, about 30 miles from the border. Some units were reported as close as 10 miles, a step likely to increase anxiety in Bangkok. The sound of heavy artillery was heard at the Thai border town of Aranyaprathet.

Part of the force that reached Sisophon apparently moved south toward Battambang, as other units moved on the city from the southeast. The airport at Battambang was reported under heavy air attack by the Vietnamese. An estimated 2,000 Cambodian troops are said to hold the city.

"Pol Pot is in a bad way," said one official. "He still has some regular and guerrilla units intact... But the Vietnamese appear likely to succeed militarily. After that, little rebel bands will likely continue forays much the same way as they are in Laos."

Several thousand Vietnamese troops are operating in Laos to shore up the government against rebel units, especially in the mountain regions. Large numbers of people from tribal regions that supplied CIA-trained forces in the past have streamed into Thailand in recent months reporting a major Viet-namese-led offensive.

Should the Vietnamese face a similar situation in Cambodia, it would tie down yet another large segment of their army, although an initial analysis in Washington is that Hanoi has sufficient economic support from the Soviet Union to take on such an endeavor.

The Vietnamese consistently have maintained that the forces operating in Cambodia are Cambodians, but Thai and Western intelligence sources place the number of Vietnamese in Cambodia at 100,000 and say they are supported by tanks, long-range artillery and jet fighters and bombers.

Heng Samrin, head of the new administration in Phnom Penh, sent a message to Vietnamese leaders yesterday, thanking them for recognizing his People's Revolutionary Council as the new government of Cambodia, the Vietnamese News Agency reported.

The agency said the message thanked Vietnam for its unreserved support in overthrowing the Pol Pot government.