Soviet and Afghan forces are attempting to expand defense lines around Kabul in anticipation of a Moslem guerrilla offensive following a Soviet pullout, Western diplomats said Tuesday.

The insurgents, meanwhile, have strengthened their own positions around the capital, moving into high ground east of Kabul, rocketing government positions to the west and laying mines on the city's outskirts, the diplomats said.

One Afghan source in Kabul was quoted saying 25 Afghan government soldiers were killed last week, when as many as four military vehicles struck mines near Kargha lake, just west of the capital.