Islamist militants bombed two Pakistani navy buses taking employees to work on Tuesday, killing four people and again bringing their war to the streets of the country's largest city. The Pakistani Taliban claimed responsibility.

Also Tuesday, Pakistan's interior minister defended the country's premier spy service against what he called defamation, a day after the release of leaked documents showing U.S. officials at Guantanamo Bay had listed the agency as a terrorist organization.

The roadside blasts took place roughly 15 minutes apart in different areas of Karachi, a southern port city that is Pakistan's economic heart. More than 50 people were wounded, Navy Commander Salman Ali said.

The Pakistani Taliban and other extremists have staged repeated attacks on security forces and other state targets in recent years. Allied with al-Qaida, they are seeking to overthrow Pakistan's U.S.-allied government or force a halt in army offensives against their bases close to the Afghan border.

Karachi, the seaside home of the Pakistani navy, has not been spared attacks, though hits in the northwest and cities in Punjab have been more common.

Police said the bombs were concealed by the side of the road, and were presumably detonated by remote control.

The victims included naval officers and employees of the force, said Ali, adding one of the dead was a female doctor.

Taliban spokesman Ahsanullah Ahsan claimed responsibility for the attacks in a telephone call to an Associated Press reporter from an undisclosed location. He said they would keep up the strikes as long as the Pakistani army continued to attack the group in the northwest.

Islamist terrorism in Pakistan began after the U.S.-led invasion of Afghanistan in 2001, which caused widespread anger in Pakistan and destabilized its border areas. Extremists began targeting the Pakistani state in earnest in 2007 after troops raided a hardline mosque in the capital, killing dozens of people.

The U.S.-Pakistan alliance has been a rocky one, with American officials often questioning whether Pakistan's intelligence services were secretly aiding Afghan Taliban groups. Prior to the Sept. 11, 2001 attacks, Pakistan backed the Taliban regime in Kabul.

According to the classified documents from 2007 released this week by the Wikieaks website, Guantanamo Bay prison interrogators were told to consider Pakistan's Inter-Services Intelligence agency as a terrorist group along with Hamas and dozens of other militant networks.

The ISI has declined to comment. But Pakistan Interior Minister Rehman Malik, while responding to questions about other subjects, praised the agency during a news conference Tuesday.

"Attempts have been made to defame ISI internationally," Malik said. "I assure you that the great work the ISI has done for this country is unparalleled. The war on terror that we are winning — the ISI has a great role and contribution in that."

Allegations of links between the ISI and Islamist militants date back to the 1980s, when Pakistan — along with the United States — was supporting the "Afghan Jihad" against the Soviet occupation in neighboring Afghanistan. These days, many analysts say the country wants to keep the militant commanders as potential allies in Afghanistan once the Americans withdraw.

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Associated Press Writer Ishtiaq Mahsud contributed to this report from Dera Ismail Khan.