Rehan Zaib Khan, an independent candidate in the Feb. 8 election who had claimed to be supported by Imran Khan's Pakistan Tehreek-e-Insaf (PTI), and four aides were shot in Bajur district, police officer Rasheed Khan said.

Rehan Zaib Khan died at a local hospital and his aides were in a critical condition, he said.

No one has claimed responsibility for Wednesday's attack in a region where Islamist militants operate on both sides of the border. It was the second killing of a candidate linked to the PTI in northwestern Pakistan's tribal regions in recent weeks.

Wednesday's attack took place a day after a bombing claimed by Islamic State in southwestern Balochistan region following a rally held there by PTI, raising security concerns amid an uptick in militant violence around the parliamentary vote.

The PTI said on Wednesday that Imran Khan and his wife Bushra were jailed for 14 years in a graft case.

The PTI says it been subjected to a military-backed crackdown, including arrests of hundreds of supporters, party members and key aides. The military, which has for decades held sway over Pakistan's politics, denies the charges.

Imran Khan's party has been stripped of its traditional electoral symbol, a cricket bat, on technical grounds, and its candidates are contesting the election as independents.

PTI's provincial president Atif Khan said Rehan Zaib Khan was a party member but the PTI had another candidate in the constituency with its official backing. In absence of the party symbol on the ballot, several independent candidates are claiming to have PTI's support.

Also Wednesday, a local leader of the left leaning Awami National Party was shot and killed by gunmen at its election office in Chaman district in Balochistan on the Afghan border, said local police official Zahoor Ahmed.

U.S. State Department expressed concerns about the violence, which it said could undermine the electoral process.

"Pakistani people have the right to choose their leader without fear for the country's stability and prosperity. Our deepest sympathies to those affected," it said in a statement.

(Writing by Asif Shahzad; Editing by Ros Russell, William Maclean)

By Mushtaq Ali