Afghan rebels have intensified a campaign of urban terrorism in the capital, Kabul, causing authorities there to extend the nightly curfew by two hours, diplomats and Afghan rebels reported here today.

One young rebel fighter who arrived here from Kabul four days ago described how a network of small cells of insurgents over the past few weeks has ambushed Soviet convoys, executed Afghan Army oficers and government officials, looted in French cultural center of typewriters and duplicating machines for their propaganda efforts, and robbed a payroll truck of $250,000 to finance their activities.

While it is difficult to verify the specific events to desbribed to a group of foreign correspondents here today, the general tenor of his stories and of the mood of Kabul matched almost exactly a separate and unrelated diplomatic report of increased rebel activities, including nightly gunbattles in the city. r

This report said "increased incidents of rebel attacks around Kabul" were the real reason that the curfew was extended Sunday by two hours and now runs from 10 p.m. until 5 a.m. Because of the rebel activities, the diplomat reported, Kabul's streets are empty after 8 p.m.

Last Thursday, the report continued, a guard outside an Afghan dignitary's home in the Shahr-i-Now area, a major shoping and residential neighborhood, was killed by gunshots from a passing car.

These reports surfaced as Afghan President Babrak Karmal paid his first visit to Moscow since the Soviets installed him in office on the strength of their 80,000 invasion troops last December. While the purpose of the trip has not been announced, diplomatic sources believe the Soviets want to use Babrak's visit to legitimize his regime.

Besides the increase of antigovernment and anti-Soviet violence in Kabul, a second diplomatic report released here today said there is "a real danger" of famine and disease striking Afghanistan this winter.

The Afghan rebel, a 29-year-old former Army officer who said he deserted soon after the Soviet invasion, called Kabul "a virtual center for training urban guerillas" and said that within three months the city will be shaken by "very sensational happenings" including the blowing up of buildings.

He said there are about 2,000 cells of 10 to 15 members each currently undergoing training and taking part in terrorist acts in and around Kabul.

Earlier this month, he said, one cell ambushed a three-jeep Soviet military convoy, killing 10 soldiers and wounding two others near the Darulaman Palace, which is being used as a headquarters by top Soviet generals in Kabul. The rebels first shot out the jeeps' tires, then attacked them with hand genades and machine guns before running off.

His account is similar to a report received here by The Associated Press from an Afghan source who has proved to be extremely reliable in the past.

The former Afghan soldier described how a week ago he personally "executed" an Afghan Army officer by ambushing him in the Kote-e-Sing neighborhood of Kabul. These was no verification of his account. Nor did he name the slain officer or tell why the assassination took place.

Similarly, he said one of the cells he is connected with assassinated two members of the pro-Soviet Parcham (Banner) faction of the Marxist Peoples Democratic Party of Afghanistan a week ago. The killings, also unverified, took place at noon on the Guzargh bridge in the Kabul suburb of Khair Khanna, near where a massive concentration of Soviet troops and armor is located.

He also said his group killed five drunken Soviet soldiers who had tried to molest an Afghan woman. They were caught swimming in a river near Kabul, dragged up to the Soofian Bridge and killed.

But, he continued, "the one that's really giving me thrills and I still enjoy it" was the recent destruction of a Soviet tank and a jeep on the Istilif Road leading north from Kabul toward the Salang Pass gateway to the Soviet Union.

He said his group had been tipped by supporters in the Afghan Army that two tanks and a jeep would be coming up the road. They hid beside the road and hit the first tank with a rocket-propelled grenade. The second tank fired back and killed one of the rebels, but they managed with heavy machine-gun fire and more rocket-propelled grenades to destroy the first tank and the jeep, he said.

"We were all laughing because the second tank ran away," he recalled.

He said most of their arms come from deserting Afghan soldiers or from Soviet troops. Some rebels, he said, get the Soviets "high" on Afghan hashish and then steal their weapons. He said they also have some Egyptian-made Soviet-style guns that they have bought in Pakistan.

Their financing, he said jokingly, "mostly comes from the Afghan government." Two months ago, for instance, they got $250,000 by ambushing a car carrying the payroll for textile workers, he said.

Two weeks ago, a five-man rebel band looted the French cultural center, taking, among other things, the telex machine because they didn't know what it was.

"Don't be angry with us," he told a group of foreign correspondents here. "We needed the supplies very badly."

Even though he is an Army deserter, the young Afghan rebel said he was side to get a false passport by fly here. He declined to give his real name or to tell what he was doing in India.

Along with his tales of rebel derring do in Kabul, he told of the continuing rivalry between different insurgent groups. In one case, two groups -- he Hezbi Islami and the United Front -- had each killed two persons in a fight when a Soviet tank appeared. They combined to fight the tank, destroyed it and then "kissed and made up -- for the time being," the Afghan said.

Meanwhile, a diplomatic source said, arguments between Babrak's Parcham faction of the ruling party and the more numerous Khalq (Masses) faction are becoming more frequent and more open in Kabul. Parcham members, he said, get short shrift in the street-corner arguments.

These tensions now are surfacing among the police, run by Interior Minister Saed Mohammed Gulabzoi, a Khalqi, and the Army, run by Defense Minister Mohammed Rafi, a Parcham member, the diplomat said.