BANGKOK -- Cambodia's Communist Khmer Rouge guerrillas have made steady political and military gains recently, seizing small towns and battling government troops only 50 miles from the Cambodian capital, Phnom Penh.

Relief workers inside Cambodia report that tens of thousands of refugees are fleeing from mountain villages in the provinces surrounding the capital and heading for temporary resettlement camps on the plains. About 20,000 refugees are reported to have fled to Phnom Penh, mostly from Kompong Speu Province, west of the capital.

Relief workers in the country and the few Western journalists who recently have visited Phnom Penh said the capital has become increasingly tense, but there were no outward signs of panic. They said several new security checkpoints have been added to roads leading from the capital to the provinces.

Diplomatic sources in Bangkok said the Khmer Rouge has stepped up guerrilla attacks in Kompong Speu and Kampot provinces southwest of the capital and in Kompong Chhnang and Kompong Thom provinces to the north, and has cut the road from Phnom Penh to Siem Reap and the historic temples of Angkor Wat.

The Khmer Rouge guerrillas appear to be following a military strategy similar to that which allowed them to seize power in 1975, after which they launched a radical social experiment that left up to 2 million Cambodians dead from murder, starvation, forced labor and disease.

After being ousted from Phnom Penh in January 1979 by invading Vietnamese troops, the Khmer Rouge joined in a coalition with two non-Communist resistance groups to battle the Hanoi-installed government in Cambodia. The Khmer Rouge has built up the largest army in the U.N.-recognized coalition, raising concern that the group might fight its way back into power. International peace efforts have thus far failed to reach a political solution to the war.

According to diplomats and other analysts, the Khmer Rouge forces are bypassing the large provincial capitals, building up administrative structures in the isolated villages they control and encircling Phnom Penh from surrounding provinces -- just as they did in the 1970s.

"The resistance has made tremendous gains on the ground," said an Asian diplomat here. "They are now tightening the noose around Phnom Penh."

"What is happening now is only a repetition of what happened in 1975," said Ek Sereywath, Bangkok spokesman for the non-Communist guerrilla group led by Son Sann. "The defense line of Phnom Penh is becoming thinner and thinner."

Despite the advances, analysts said, the guerrillas do not seem capable of seizing the capital.

The Khmer Rouge, with an army estimated at about 40,000 men, has benefitted from the current rainy season, which favors guerrilla warfare and prevents Cambodian government troops from moving heavy tanks and artillery.

The guerrillas also are taking advantage of apparent disarray in Phnom Penh, where Hun Sen, prime minister of the Vietnamese-installed government, is believed to be beset by fighting within the Cambodian Communist Party and rapid economic deterioration caused by corruption and cuts in aid from Eastern Europe.

The Khmer Rouge troops also are cooperating on the battlefield more closely than ever with the two non-Communist groups, particularly that of Prince Norodom Sihanouk. "All the fights are coordinated now," the Asian diplomat said.

The military cooperation between the resistance groups has become a sensitive issue in the U.S. Congress, where the House last week approved up to $7 million in non-lethal assistance -- mostly medicine, uniforms, tents and some training to the non-Communists -- provided that none of the aid benefits the Khmer Rouge.

In addition, the Senate Intelligence Committee voted to cut off covert arms supplies to the non-Communist resistance forces because of concern over their increasing military involvement with the Khmer Rouge.

Officials of Sihanouk's faction here conceded that the number of cooperative guerrilla operations has increased, particularly in Kompong Thom Province. But they said the cooperation between the two non-Communist groups and the Communist Khmer Rouge actually began in 1982, when the feuding Cambodian factions were forced into an unlikely coalition with Sihanouk as its nominal head.

"Cooperation does not mean merger," Prince Norodom Ranariddh, Sihanouk's son, said in an interview here. "We have our own uniforms. We have our own military structure for command and control. We are very proud."

Ranariddh, who commands the military wing of the group headed by his father, and Ek Sereywath, spokesman for the other non-Communist group, said the non-Communists often have no choice but to cooperate with the Khmer Rouge on the battlefield because the Khmer Rouge is the largest army and would join in attacks even if not invited.

Referring to a joint attack June 4 to seize the district capital in Kompong Thom Province, Ek Sereywath said, "We could not prevent the Khmer Rouge from attacking with us. We could not say, 'No, don't attack, let us do it.' How can we prevent them from . . . doing their nationalist duty?"

Ranariddh said the main issue now was not the extent of his group's cooperation with the Khmer Rouge, but rather how to prevent the Khmer Rouge from operating in too many places alone. He said the non-Communists can act as a check on the Khmer Rouge by moving in to jointly administer "liberated" villages to prevent the Khmer Rouge from building a separate political base.

Ranariddh and other diplomats said the Khmer Rouge already is building a substantial following in the areas it now controls in southwestern Cambodia by shunning violence and attempting to project a benevolent image.

"Now they don't kill," Ranariddh said. "They go there with textiles, medicine, even videotapes. It is more dangerous than AK-47s." Ranariddh said Khmer Rouge cadres often arrive in "liberated" villages with truckloads of supplies for the peasants.

"They are building up their own political and administrative structures, expecting free elections. And they are doing it alone," Ranariddh said.

Other analysts here agreed with Ranariddh's view that the Khmer Rouge has made significant political gains. "The Khmer Rouge are successfully increasing their influence in the countryside," said a Western diplomat here who monitors Cambodia. "They're not going into villages raping women and killing children. What they are doing is appealing to crude Cambodian nationalism. They are not corruptible, and they are not being particularly violent."

Ranariddh said the only way to "neutralize" the Khmer Rouge is for the non-Communists to get help in building their own administrative infrastructure in areas now under non-Communist control.