Four people were killed when a suicide bomber posing as a legal clerk blew himself up and his companion opened fire at a courthouse in northwest Pakistan on Monday.

Four people were killed when a suicide bomber posing as a legal clerk blew himself up and his companion opened fire at a courthouse in northwest Pakistan on Monday, officials said.

The attackers stormed the crowded court complex in the city of Peshawar, less than two months before expected national elections.

Local TV footage showed shards of glass and broken window scattered on the floor of a court building with shattered cabinet doors.

"One suicide bomber blew himself up in the court of an additional sessions. One other man was shot dead by police," said senior police officer Masood Khan Afridi.

"It was an act of terrorism and the target was the judicial complex," he added. "We have cleared the whole area."

Afridi denied reports that some judges and lawyers had been held hostage inside the courts.

"Terrorists have attacked at a time when general elections are very near and the atmosphere for election is smooth," said the information minister for Khyber Pakhtunkhwa province, Mian Iftikhar Hussein.

The attack is the second in Peshawar in less than a month. Militants including a suicide bomber attacked the office of a senior official there on February 18, killing six people.

The heavily-guarded court complex is in front of a five-star hotel and close to government and official buildings.

Khyber, part of Pakistan's semi-autonomous tribal belt on the Afghan border, is in the grip of intensified fighting as part of a long-running military operation against the Taliban and other Islamist insurgents.