ISLAMABAD, PAKISTAN, DEC. 25 -- An Afghan rebel leader today acknowledged that Soviet and Afghan government troops had made headway in a push to reach their besieged forces in Afghanistan's eastern city of Khost.

Younis Khalis also dismissed Soviet claims there were 1,500 guerrilla casualties when the push began last week. He said that in fighting near Khost today, 45 government troops surrendered to guerrillas and 10 Soviet tanks were destroyed.

"We accept that they advanced a few kilometers, about 10 to 15 kilometers {6 to 9 miles} from their former positions," Khalis said in a telephone interview from the Pakistani border city of Peshawar.

The official Soviet news agency Tass, meanwhile, claimed today that a group of former rebels has handed over a West German weapons instructor to the Soviet-backed Afghan government. The dispatch did not say when or where the man was captured.

Tass identified him as Osman Demir and said he worked for West German intelligence. The agency said Demir had trained rebels in using U.S.-made Stinger missiles. Khalis, leader of the seven-party Afghan opposition alliance, denied the report.

Khalis said government forces had advanced only "because we cannot destroy all their tanks and planes all at once."

He added, however, that the number of Moslem fighters participating in the battle for Khost was increasing.