Afghan Moslem guerrillas have blocked all roads to the city of Jalalabad, where Soviet troops are still based, an alliance of guerrilla parties said.

The guerrilla strategy, which is emerging from reports of front-line action during the Soviet withdrawal from Afghanistan, appears to be to allow powerful Russian columns to leave garrisons and then close in on the weakened forces left behind.Western diplomats said many small towns and villages have fallen to the guerillas in this way, but if Jalalabad also fell it would be a severe blow to the Soviet-supported government of President Najib.

The first Soviet troops to start withdrawing from Afghanistan under the terms of an international agreement left Jalalabad, which controls the eastern approaches to Kabul, on May 15.

A column of 1,200 soldiers from the city crossed into the Soviet Union on May 18 after traveling through Kabul.

But there are apparently still Soviet troops in Jalalabad. On May 15, the commanding Soviet officer told reporters that it would be two weeks until the last Soviet soldier left the city. A spokesman for the Islamic Alliance said from its headquarters in the Pakistani border town of Peshawar that guerrillas have cut the only major east-west road that passes through Jalalabad.