High-profile Sufi assassination
Pakistan Army spokesman Lieutenant General Asim said 20 ambulances and 50 soldiers had reached the site, while a further 45 ambulances 100 troops were on their way.
A military helicopter would attempt evacuations at night, he added, but medical teams could not access the area by plane as their were no air strips close by.
Four bodies and 21 of the injured had arrived in Karachi's Civil Hospital, officials said.
President Mamnoon Hussain and Prime Minister Nawaz Sharif strongly condemned the bombing in separate statements.
"The government is determined to eliminate terrorism and extremists from the country," Hussain said in a statement expressing sympathy with the victims and their families.
A statement from Sharif's office said the prime minister called for the "best medical treatment" to be given to the wounded.
Up to 600 people were at the shrine at the time of the attack, according to local official Tariq Mengal, who told Geo TV that many devotees travelled to the site from Karachi during weekends.
The bombing follows the killing of Amjad Sabri, a renowned Sufi singer, by two gunmen in Karachi in June.
Some observers have said that Sabri may have been assassinated because he was a high-profile Sufi.
Sufism, a mystic Islamic order that believes in living saints, worships through music, and is viewed as heretical by some hardline groups including the Taliban.
Balochistan, which borders Iran and Afghanistan, has oil and gas resources but is afflicted by Islamist militancy, sectarian violence between Sunni and Shiite Muslims and a separatist insurgency.
Local militants claimed to have worked with the Islamic State group to attack a police academy in Balochistan last month, killing 61 people in the deadliest assault on a security installation in Pakistan's history.
In August, a suicide bombing at a Quetta hospital claimed by IS and a faction of the Pakistani Taliban killed 73 people.
