ISLAMABAD, Pakistan — Pakistan is sending in troops against hundreds of militants who overran the district of Buner last week, the government announced Tuesday afternoon, saying that the Taliban would not be allowed to expand their territory.

A military spokesman announced that troops had completed a two-day operation against militants in Dir and started a new operation on Tuesday afternoon in the district of Buner at the request of the government.

Paramilitary forces of the Frontier Corps, backed by army helicopters and fighter jets, were involved in the operation, he said. The fighter jets were being used to clock the entrances to the valley, he said. He estimated there were 450 to 500 militants in the district of Buner and said the Taliban had staged a show withdrawal at the weekend and were in fact trying to expand the space they control beyond the valley of Swat.

“In Buner people are living under coercion and in fear,” he said. “There was no reason to intimidate people in Buner and the militants started intimidating people and forcibly recruiting young people to take them back to Swat for military training,” he said.

“The government acted with patience but eventually there was no other way except to launch an operation,” he said.