BENGHAZI, Libya — Libyan rebels have broken off their assault on a key city south of Tripoli after discovering that forces loyal to ousted dictator Moammar Gadhafi there had placed Russian-made Grad rockets and mortars on the roofs of houses filled with civilians, the rebels' military spokesman said Sunday.

Col. Ahmed Omar Bani said the decision to halt the rebel offensive on Bani Walid, where Gadhafi's son Saif al Islam is believed to be hiding, made it unlikely the rebels would have full control of the country before the end of September.

The decision to retreat from Bani Walid came as news agencies reported that another of Gadhafi's sons, Saadi, had arrived in Niger, Libya's neighbor to the south.

The decision to retreat from Bani Walid also upset the timetable for taking control of Sirte, Gadhafi's hometown, and Sabha, a loyalist-held city deep in Libya's southern desert. Abdul Hafez Ghoga, the civilian spokesman for the council, had predicted that Bani Walid would fall this weekend and Sirte by the end of the week.

Bani's update made it clear that that timetable was now impossible.

"The most important thing is to liberate our country," Bani said. "By the end of the month, we will."

Bani said the rebels would maintain their siege of Bani Walid, a town of 70,000 about 100 miles south of Tripoli, while waiting for supporters inside the city to mount operations that would change the situation.

"NATO can do nothing," Bani said of the North Atlantic alliance's airpower, which has proved decisive in the rebels' advances since they began their revolution Feb. 17.

Bani accused the loyalists of shooting Bani Walid residents who try to escape. "Instead of killing 70,000 in Bani Walid, we prefer...to surround the town," he said.

At least three rebels were reported killed in the initial move into the town Friday night. Bani declined to give details of casualties, except to say that "it's not a big number."

Rebel spokesmen also have accused Gadhafi loyalists of using prisoners as human shields in Sirte.

Bani said Sabha remains under the control of forces composed largely of non-Libyan fighters.

But he predicted that three small towns held by loyalists in the Jufra oasis, about half way between Sirte and Sabha, would fall in coming days. Hun, Sokna and Waddan had all been given an ultimatum Friday to surrender or face a military assault by Sunday night. Bani predicted that at least one of these towns would fall Sunday or Monday.

NATO has attacked arms depots and surface-to-air missiles in these three towns in the past week.

(Special correspondent David Enders contributed from Tripoli)

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