A series of sectarian massacres on one of Pakistan's most important roads has forced the government to lay on military flights for Shias travelling to and from the country's capital.

The decision to allocate C-130 Hercules transport planes for the use of civilians travelling between Islamabad and the north-eastern provincial capital of Gilgit follows the killing on Thursday of 20 Shias.

In the third such incident on the road in six months, the victims were pulled off a bus some 100 miles north of Islamabad by armed men, lined up and shot.

Local officials said up to a dozen people wearing army uniforms had stopped the bus before mounting their attack.

"After checking their papers, they opened fire and at least 20 people are reported to have been killed," said Khalid Omarzai, administration chief of Mansehra district.

Pakistan is struggling to deal with a rising tide of sectarian violence and extremist Sunni militant groups who do not regard members of the Shia sect as true Muslims.

The prime minister's office said the flights were a temporary measure as the country heads towards the Islamic holiday of Eid al-Fitr.

"Normally people would travel by road, but because of the situation people are no longer prepared to use it," a spokesman for Raja Pervaiz Ashraf, the prime minister, said.

The road is critical for connecting Gilgit-Baltistan, the mountainous province that borders China, with the rest of the country. With stunning scenery and access to some of the world's highest mountains, it was once popular with international tourists travelling the Karakoram Highway.

But Gilgit and its surrounding area have been repeatedly hit by appalling sectarian violence.

In February, 18 Shias were forced off a bus travelling to Gilgit and killed. And in April nine travellers were shot dead in a town 60 miles from the city.

In another attack on Thursday a bomb appeared to target a bus carrying Shia students to an anti-Israel rally in Karachi, the megacity on the country's southern coast, which is also frequently racked by sectarian killings.

Police said one man was killed and 11 were wounded as the bus travelled to the protest held annually at the end of the fasting month of Ramadan.

Many analysts have criticised the Pakistani government for not doing enough to suppress anti-Shia groups. The government and military are often reluctant to launch crackdowns in the Sunni-majority country, in which radical groups enjoy mainstream support from the public.