Senior surveillance officer Sarfaraz Tan Afridi, responsible for the WHO team in the North West Frontier Province, said his group recorded the deaths in recent days. "We are on alert," he said.
"We are trying to get as many people immunised as soon as possible."
Tetanus, more commonly known as lockjaw, is a bacterial infection of the nervous system that can cause fever, high blood pressure and severe muscle contractions.
It can lead to the "locking" of the jaw, general stiffness, and death, especially among the elderly.
People become infected when the bacteria, found in the ground and in faeces, enter through cuts or scratches.
"It's a public health concern," WHO spokesman Chris Black said in Islamabad. But he added that the "disease is completely preventable through vaccinations."
A day earlier, WHO had said it planned to send in 100,000 doses of tetanus vaccine within 48 hours.
Dr Khizer Hussain, who has been volunteering in the Balakot area, said he had been vaccinating everyone who came in for treatment.
"Forth-eight hours is usually enough for tetanus to set in. People are coming in days later," he said.