Forces loyal to the ousted Khmer Rouge Premier Pol Pot have regrouped with unexpected speed to launch large scale attacks with some success against the Vietnamese divisions in Cambodia.

Although analysts here do not entirely discount claims over Khmer Rouge radio today that Pol Pot's forces raided within 12 miles of the capital, the small scale fighting around Bek Chan -- renowned among the last ditch defense attempts of the U.S.-supported Lon Nol army in 1975 -- is thought as yet to pose no serious threat to Vietnamese control of Phnom Penh.

The fighting around the provincial capitals of the south and west, however, is considered must more serious. The Khmer Rouge radio -- thought to be broadcasting from Chira's Yunnan Province -- yesterday claimed the recapture of the whole southwest, with the exception of Kampot where Vietnamese troops remain bottled up.

Although analysts consider these reports exaggerated, independent information indicates that the Khmer Rouge have managed to restore the integrity of their army with at least two military headquarters communicating with tactical units and coordinating effective operations against the Vietnamese.

One military source estimates that four-fifths of the Cambodian army -- or 60,000 men -- have remained intact and regrouped in large units, mostly brigades of roughly a thousand men each, responding to central commands.

Casualty figures for both sides are vague. But in some engagements the Vietnamese are known to have lost hundreds of men.

The pro-Hanoi Cambodian government of Heng Samrin in Phnom Penh claimed 10 days ago that its forces "totaliy control all the country's territory" and denounced "distorted foreign press reports" of the Khmer Rouge guerrilla advances as the "propaganda trick of a number of big countries... who support Pol Pot in order to expand the war." The reference to "big countries" is considered by observers here to mean China and the United States.

But if the reports on which diplomats here base their views are accurate, the 14 full Vietnamese divisions now esthnated to be in Cambodia appear to be more thinly spread and begged down than first implied by the lightning victory of their 14-day assault to capture Phnom Penh.

None of this threatens the initial gains from Vietnam's swift and deep thrust to capture Cambodia's key towns and communications. But as the Khmer Rouge army revives more intact than most assumec, the cost to Victnam of holding the captured ground will increase.

"It now looks like a far longer, bloodier and more expensive operation than Hanoi probably counted on," one observer said. Hanoi may have little choice but to pay an unexpected high price.

The assault on Takeo, the provincial capital 50 miles south of Phnom Penh, is the first major operation by the Khmer Rouge since they evacuated Phnom Penh three weeks ago. Apparently catching thin Vietnamese security forces in the twon unprepared, the Khmer Rouge recaptured Takeo Jan. 23 with two brigades of in fantry reinforced by artillery and armor.

The Vietnamese managed to recover Takeo only recently after several days of intensive fighting and heavy air attacks which destroyed seven Khmer Rouge tanks and several heavy vehicles, analysts here say. The surrounding countryside continues to be heavily contested.

Similar operations were simultaneously launched against the coastal towns of Kampot and Ream, which the Khmer Rouge have managed to capture and hold until now. Although the Vietnamese have regained the strategic deep-water port of Kampong Som after losing if twice, the Khmer Rouge have cut Highway 4 running between the port and capital in at least three places.

Closer to Phnom Penh, the Khmer Rouge have launched small night raids penetrating into the market place at Kampong Speu, 45 miles southwest of Phnom Penh. One intelligence report claims the Khmer Rouge slipped into Pochentong airport and managed to steal significant amounts of gasoline.

Reliable reports sketch a similar picture in western Cambodia. The Khmer Rouge, in large-scale engagements, have pinned down the Vietnamese in Kampong Chhnang, Pursat and Battambang -- three provincial capitals northwest of Phnom Penh along Highway 5. A Vietnamese armored column heading southwest toward the Thai border from Battambang has been stalled for over a week by a heavy artillory defense line, as yet unbroken.

Another report indicates that the Khmer Rouge massed in enough strength last week to surround a Vietnamese regiment and pin it down somewhere in the south west.

By evacuating the main towns and dissolving into small units the Khmer Rouge appear to have "jerked the anvil out from under the Vietnamese hammer blow," in the words of one veteran Indochina analyst.

Three weeks after the capture of Phnom Penh the Vietnamese remain heavily road-bound. Even their control over the roads remains tenuous and vulnerable to daylight attack.

The garrisons in the west are particularly exposed at the end of everstretched supply lines. The Khmer Rouge have blown bridges and trenched roads behind the Vietnamese advance.

Recently rice has had to be airlifted into Battambang and Siem Reap -- the traditional granaries of Cambodia -- a good indication that the Vietnamese either cannot move into the countryside or find no food when they arrive.

So far the Khmer Rouge appear to have denied their enemies the stocks of fuel and food the Vietnamese appear to have been counting on.