BANGKOK, Thailand (AP) _ Communist Khmer Rouge guerrillas are preventing Cambodian refugees from going to a U.N. camp and instead are moving them toward an area that has been a combat zone, U.N. officials said today.

Also today, a Thai military spokesman said Cambodian guerrillas have cut parts of important government supply lines from the Cambodian interior to battlefields in the west.

The sound of gunfire crackled in the isolated Borai camp shortly after U.N. trucks arrived there Monday to take refugees to the Site K camp, said Patrick Van de Velde, deputy director of the U.N. Border Relief Operation.

The 10 trucks remained outside Borai today, and repeated shooting appeared designed to stop the refugees from leaving, said another relief official, who spoke on condition of anonymity.

Borai, run by the Khmer Rouge, is near Thailand's eastern province of Trat and a few miles from Site K.

None of the Borai refugees had boarded the trucks by today, although some refugees in the past have pressed relief officials to be transferred to Site K.

''They are too scared to leave,'' said another aid official, also speaking anonymously.

There was no statement from the Khmer Rouge.

He said camp authorities had ordered the refugees to head toward areas along the Cambodian frontier recently secured by the guerrillas. Since Jan. 15, the Khmer Rouge have moved about three-fourths of 4,000 to 4,500 people at Borai to the border area, the aid official said.

Urs Boegli, head of the Thailand office of the International Committee of the Red Cross, said: ''The danger is they are being moved toward a combat zone.''

The Khmer Rouge is the largest guerrilla group fighting the Vietnamese- instal led government in Cambodia.

During a 3 1/2 -year rule that was ended by Vietnam's invasion in late 1978, the Communist fanatics slaughtered hundreds of thousands of people in an attempt to radically restructure Cambodian society.

The guerrillas took tens of thousands of civilians along as they retreated to the Thai border in 1979, and widespread human rights violations have been reported in its civilian camps.

The Khmer Rouge have resisted moving refugees to U.N.-aided camps for fear of losing control over them. But amid mounting international pressure, the guerrillas in the past year have let nearly 30,000 refugees from secretive military-run camps resettle in two U.N. camps, O'trao and Site K.

This had raised hopes for transferring the people from Borai, the most hard-line of four Khmer Rouge camps where the United Nations has had access.

Just last week, after months of negotiations, Borai administrator Lok Tia assured a U.N. official the refugees could freely decided whether to go to Site K.

A coalition of three guerrilla factions is fighting Cambodia's Vietnamese- ins talled government.

The coalition - including the Khmer Rouge and the non-communist forces of Prince Norodom Sihanouk and the Khmer People's National Liberation Front - has made major gains since late September, when Vietnam withdrew what it said were its last troops in Cambodia.

At a press briefing in Bangkok today, Thai military spokesman Rear Adm. Kraichit Sirisombat confirmed guerrilla claims they cut parts of Route 5 and Route 6, which run the length of Cambodia to the western border with Thailand.

Government forces now were concentrating on securing the most important cities, he said.