Pakistani troops patrol Friday in the Bajour Agency, Pakistan. Pakistan has decried this week's exchange of fire between U.S. and Pakistani forces.

Washington ? The nation’s top military officer tried Friday to tamp down tensions surrounding the escalating violence along Afghanistan’s southern border, including this week’s exchange of fire between U.S. and Pakistan forces.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said Pakistan’s military leaders reassured him in talks there last week that they have no intention of using force against U.S. troops along the Pakistan-Afghanistan border.

“Things are very tense and very dangerous in Pakistan,” Mullen told a Pentagon news conference. “But that doesn’t mean the sky is falling :. Now more than ever is a time for teamwork, for calm.”

Mullen’s remarks came as Pakistan’s president walked a fine line, saying Friday that he still looks positively on U.S. support to his nation despite the Thursday dustup on the border. But Asif Ali Zardari also warned the U.N. General Assembly on Thursday that Pakistan cannot allow its territory to “be violated by our friends.”

The five-minute firefight Thursday underscores the murky nature of the relations between the U.S., Pakistan and Afghanistan and how they interact along what is a long, mountainous, ungoverned border riddled with safe havens for Taliban and al-Qaida insurgents.

Pakistan has become increasingly vocal in its anger over U.S. operations across the border, including a spate of missile and ground attacks aimed at insurgents in the tribal areas. U.S. military commanders complain that Islamabad has been doing too little to prevent the Taliban and other militant groups from recruiting, training and resupplying in the border region.

Mullen acknowledged that the safe havens have gotten safer this year, and the insurgency more sophisticated. And he has launched a review of the military’s strategy in Afghanistan insisting that it also focus more broadly on Pakistan and even India, because the three are interwoven and must be dealt with in a comprehensive way.

Thursday’s clash began when Pakistanis fired on or sent flares at two American reconnaissance helicopters operating near the border. The Pakistanis said the U.S. choppers, which were escorting Afghan and U.S. troops, had crossed into the tribal Pakistani areas, but Pentagon officials flatly deny that.