A bomb hit troops from the U.S.-led coalition patrolling south of the Afghan capital Thursday, causing an unspecified number of casualties, the coalition said.

In neighboring Pakistan, the government denied that its main spy agency was behind a recent assassination attempt on the Afghan president and took aim at Kabul for failing to prevent the attack.

The bomb struck the coalition convoy on Thursday morning during a patrol in Wardak province, a coalition statement said. Troops secured the scene, but "the exact number and nature of casualties" was not immediately clear, it said.

A freelance television cameraman filmed what he said was the aftermath of the Thursday attack. The footage showed the burning wreckage of a vehicle on a bend in a mountain road. Militants held up what looked like an M-16 rifle and dragged away a belt of ammunition.

It was not possible to independently verify whether the footage was from the same incident reported by the coalition. The cameraman's name was withheld for his own security.

Afghan officials confirmed the attack but provided no details.

Fighting between Taliban-led insurgents and security forces continues unabated, despite a nearly seven-year international effort to stabilize the Western-backed government of President Hamid Karzai.

More than 2,000 people have died in insurgency-related violence this year, according to an Associated Press tally, including at least 111 foreign soldiers.

Much of the fighting has taken place in the eastern and southern provinces bordering Pakistan.

Afghan leaders accuse Pakistan of secretly supporting the insurgents and harboring their leaders _ a charge Pakistan civilian and military leaders deny.

In the latest and most serious allegation, an Afghan official on Wednesday blamed Pakistan's Inter Services Intelligence agency for an attempt to assassinate Karzai during a military parade in April.

Saeed Ansari, spokesman for the Afghan intelligence service, said the confessions and cell phone records of detained suspects and other unspecified evidence proved the ISI was the "main organizer" of the assassination attempt.

Pakistan's Foreign Ministry on Thursday rejected the allegation as "baseless and irresponsible."

Ministry spokesman Muhammad Sadiq suggested Kabul was trying to divert attention from a "massive intelligence and security failure" of its own.

Confidence in Afghan security forces was further shaken by a June 13 Taliban attack on the prison in the southern city of Kandahar, which freed 400 Taliban fighters.

The Interior Ministry said Thursday it had fired three senior police officials including Kandahar provincial police chief Sayed Agha Saqib and referred his case to the prosecutor's office.

A ministry statement said Saqib was "negligent in his duties," but it did not mention the jail break or what charges he might face.

In other violence, the coalition said warplanes attacked insurgents who fired on Afghan and U.S.-led forces on patrol in the Maywand district of Kandahar province on Wednesday.

The insurgents were killed with "several precision airstrikes," it said.

No government or coalition troops were reported injured.

The coalition also said that a helicopter made a "precautionary landing" in the rocky eastern Afghan province of Kunar on Wednesday.

None of the troops on board suffered serious injury and all returned to their base, it said.

There were no reports of enemy fire forcing the chopper down.

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Associated Press writers Rahim Faiez in Kabul and Asif Shahzad in Islamabad contributed to this report.