Pakistan's military has blockaded a strategically important district in the country's north, sealing in a fiercely anti-Taliban tribe.
The Turi people have been keeping Taliban militants out of Kurram tribal district, near the Afghan border.
Many in Kurram suspect the government is pressurising the Turis to meet Taliban demands to cross their land.
Any deal between the Turis and the Taliban could have major implications for Nato's operations in Afghanistan.
The Turis, who follow the Shia branch of Islam, have traditionally abhorred the Taliban, who adhere to a hardline Sunni form of the faith. Many Taliban consider Shias to be non-Muslims.
The blockade comes amid reports that the Turis have once again refused to allow the militants to enter Afghanistan via Kurram.
The Taliban have been trying to launch operations around Kabul through the district, whose western tip lies just 90km (56 miles) from the Afghan capital.
The blockade means that the Turis are hemmed in by the military on one side and by the Taliban on the other.
Col Tausif Akhtar, of the Pakistani security forces, announced the move on Monday evening at a news conference in Parachinar, the main town in Kurram.
Five border crossing points - Terimangal, Spina Shaga, Khairlachi, Burki and Shahidano Dand - have been shut, with security beefed up.
"We have done this due to internal security concerns, because there have been sectarian clashes in Kurram and we do not want miscreants from outside to exploit the situation," Col Akhtar told the BBC News website.
But the BBC's M Ilyas Khan in Islamabad, one of the few journalists to have travelled to Kurram in recent months, says the move is baffling.
The Turis have kept members of the Haqqani network - a branch of the Afghan Taliban based in Pakistan - away from Kurram.
The tribe beat the militants out of the area during a major battle in September 2008.
In retaliation, the Taliban have blockaded the east of Kurram, effectively cutting off the area from the rest of Pakistan. The militants have been ambushing Turi commuters along a 10km stretch of road.
The tribe has been forced to rely on trade with Afghan towns and villages over the border.
But the government decision to block this route, too, places the Turis under an economic stranglehold, says our correspondent.
Haqqani network members last week held talks with Turi leaders in Islamabad about striking a deal for access to Kurram.
In return, the Taliban is thought to be offering safe passage for Turis travelling overland from Kurram to Peshawar.
But the Turis reportedly rejected the Taliban approach - for at least the fourth time since 2008.