QUETTA, Sept 5 (Reuters) - At least three Pakistani paramilitary soldiers were killed and 20 wounded in a suicide attack in the southwest on Sunday, officials said, part of a spike in attacks on security forces in recent weeks as neighbouring Afghanistan fell to the Taliban. The militant Tehrik-e-Taliban Pakistan (TTP) claimed responsibility for the attack. The group, which is separate from the Afghan Taliban, renewed its allegiance to that group after the fall of Kabul last month and has recently stepped up its campaign against the Pakistani army. Islamabad fears a rise in militant attacks along the Afghan-Pakistan border as the Afghan Taliban tries to fill a vacuum left by the collapse of the Western-backed administration in that country. The attack occurred as a checkpoint security detail was being relieved, said Sheikh Rashid, Pakistan"s interior minister. Three members of the Frontier Constabulary (FC) were killed and 20 wounded, he told reporters. A suicide bomber on a motorcycle packed with 6 kg (13 pounds) of explosives rammed a vehicle in an FC convoy, said Azhar Akram, deputy inspector general of the Quetta police. “Condemn the TTP suicide attack on FC checkpost in Mastung Road, Quetta,” Prime Minister Imran Khan said in a tweet. A senior Pakistani official said last week that Islamabad is worried about militant fighters from the Pakistani Taliban group crossing from Afghanistan and launching lethal attacks on its territory. The Afghan Taliban have sought to reassure their neighbour that they will not allow their territory to be used by anyone planning attacks on Pakistan or any other country. Sunday"s attack took place in Quetta, the capital of the southwestern Pakistani province of Balochistan, about 100 km (60 miles) east of Afghanistan. Separatists have fought a low-level insurgency against the Pakistan government for decades in Balochistan, where China is increasing investment in energy and infrastructure projects.