Israel has begun turning its military checkpoint on the western outskirts of this central Golan Heights town into an international border crossing since its decision a month ago to annex its occupied portion of this strategic Syrian territory, U.N. officials said here today.

The move comes in spite of the recent U.N. Security Council resolution unanimously condemning the Israeli annexation and has raised tensions even higher between the two enemy nations.

U.N. officials said, however, that there was no sign of any new military buildup by either side over the latest Israeli action. Construction of permanent buildings and an enlarged parking area for inspecting vehicles at the Israeli checkpoint started about a week ago, according to U.N. officials, who are concerned that the move might hamper their travel across the disengagement lines established by the 1974 Geneva peace conference.

It coincided with the start of a U.N. Security Council debate on what measures the international community might impose on Israel in retaliation for its annexation and is apparently being done to affirm Israeli sovereignty over the Golan Heights up to the outskirts of Quneitra.

The Syrians are reported to be furious aboutthe Israeli border post construction, although so far they have made no public statement about it. From the last Syrian checkpoint on the western edge of this war-devastated town, an Israeli bulldozer was visible 300 yards away today leveling the land around the new buildings.

"They have told us they were going to make it into an international frontier crossing," said Lt. Col. James Allan, the Canadian deputy chief-of-staff of the U.N. Disengagement Observer Force. "They are putting up new buildings to replace the old corrugated shacks."

Allan said he understood the Israelis intend to build a border post modeled on that at El Arish in the Israeli-occupied Sinai where travelers to and from Egypt cross. But there is no civilian or military traffic across the Syrian-Israeli disengagement lines here and the only group that could be affected by the Israeli move is the 1,288-man U.N. force stationed within the 50-mile-long demilitarized zone separating Syrian and Israeli forces on the Golan.

So far there have been no problems, according to Allan. But he is not sure whether Israel, to establish the principle of its sovereignty and the idea of the crossing here as the limits of its territory, will impose new restrictions on the passage of U.N. personnel across the lines.

U.N. officials fear that the Syrians also may act to prevent the U.N. peacekeeping personnel from doing anything that might imply international acceptance of Israeli jurisdiction at the Quneitra crossing.

Half of the U.N. disengagement force monitors Israeli compliance with the 1974 Geneva agreement and the other half monitors Syrian compliance. Its headquarters is in Damascus but its main logisitical unit is on the Israeli side. This requires constant crossing by U.N. personnel of both the Israeli and Syrian checkpoints here.

The demilitarized zone varies in width from 300 yards in the south near the Jordanian border to about 10 miles in the middle and it contains 22 observation posts and 33 other positions manned by U.N. forces. There are limits on the number of troops and heavy arms that can be stationed within 15 miles of the demilitarized zone.

Allan said that except for a brief and quickly corrected violation by Israel of these limitations during its initial buildup immediately after the Dec. 13 annexation, both sides had respected the agreement.

"It has been very, very calm," he said.

Syria has left Quneitra a scene of total destruction as they say they found it upon the Israeli withdrawal in June 1974. Hardly a house remains standing and it is the Syrian contention, backed up by a U.N. report, that the Israelis used bulldozers and dynamite to level the crossroads town before they gave it back to the Syrians.

The Syrian government now uses it as a showpiece for visitors in its campaign to discredit Israel and prove the Israelis' bad intentions toward their Arab neighbors.

The destruction is made more stark by the contrast of green fields and whitewashed homes of two Israeli settlements visible just outside Quneitra. Syria has allowed fewer than 50 people to remain living in the ruins of the old town. There is a school, for the 14 children of these families plus 17 from nearby villages, in the remains of the municipal building.

One of the residents is Amina Hassan Jokh, a wrinkle-skinned and largely toothless old woman with a twinkle in her eye and a defiant voice. She lives with her son and his family in one of the few homes still standing in the town's center.

Amina, a Circassian, recounted to two visiting Western reporters her experiences since her birth here "75 or more years ago." She said she had stayed in the town through both the 1967 and 1973 Arab-Israeli wars and saw it exchanged four times between Israeli and Syrian forces.

"I was not afraid of the Jews," she said, recounting how she had watched Israeli soldiers in June 1974 set fire to two of the nearby houses while she stood in the doorway defending her own with a knife in hand.

She said Syrian President Hafez Assad had personally asked her to remain in the house where she lives now when he was on a visit here shortly after its return to Syrian hands.

Meanwhile, Syria has begun building a new town a few miles away on the road to Damascus as well as nine villages along the disengagement line to house the 140,000 Syrians evacuated from the war zone during the 1973 fighting.