In a rare admission, Afghan President Najibullah said government forces had lost control of the southern garrison town of Khost and declared Tuesday a day of mourning for those killed defending it.

Afghan guerrillas seized Khost on Sunday after a 17-day battle. The town is located 18 miles from the Pakistani border."Today . . . our links with Khost were cut and many of our brave sons received martyrdom," Najibullah said late Monday in a Kabul radio broadcast monitored in New Delhi. "Therefore, I declare . . . a national day of mourning."

Najibullah blamed deep snows and the resulting bad road conditions for the loss of Khost. And he accused Pakistan of directly helping the guerrillas. Islamabad denies the charges.

Pakistan is home base for several Muslim guerrilla groups that have been trying to overthrow successive communist-style Afghan governments for nearly 13 years.

Khost, with a prewar population of 15,000, was the government's most far- flung outpost. The town is on major supply routes connecting Kabul, the Afghan capital, and southwestern provinces and had long been a symbol of strength for the government.

One Western diplomat, speaking on condition of anonymity, said the guerrilla victory did not spell control of the country.

"The mujahedeen (guerrillas) can capture Kandahar, Kunduz, Herat, Jalalabad and every other major city in Afghanistan and it will mean very little. They still won't hold Kabul, and those who control Kabul control Afghanistan," the diplomat said.