ISLAMABAD - Pakistan's military said yesterday that its ground forces exchanged fire with a NATO helicopter in another possible flashpoint with Washington, but also claimed it arrested a senior al Qaeda operative following U.S. demands for "actions, not words" to restore trust.

The two reports highlight some of the complexities of trying to rebuild ties after the U.S. raid that killed Osama bin Laden earlier this month. Washington needs Pakistan as a crucial partner against al Qaeda, but Pakistani officials remain deeply angered by the secret operation over their borders in the assault on bin Laden.

In a possible sign of stronger controls on the frontier, Pakistani ground forces traded fire with a NATO helicopter on the Afghanistan-Pakistan border, wounding two Pakistani soldiers, officials said. The Pakistani army filed a protest, and a NATO spokesman said that an "incident" occurred at the border and that an investigation would be launched.

Pakistan's powerful army and intelligence agencies have faced uncomfortable international scrutiny since bin Laden was killed inside a fortified compound in the army town of Abbottabad.

The army said it had arrested Yemeni national Muhammad Ali Qasim Yaqub - also known as Abu Sohaib Al Makki - who they claim had been working directly under al Qaeda leaders along the border between Pakistan and Afghanistan.

On Monday, U.S. Sen. John Kerry met army and civilian leaders in Pakistan during the first visit by an American emissary since bin Laden was killed. With some American lawmakers calling for U.S. aid to Pakistan to be cut, he told them that "action, not words" were needed to tackle militancy.

The NATO firing incident took place in the Datta Khel area of the North Waziristan tribal region, a known sanctuary for Taliban and al Qaeda militants that launch attacks inside Afghanistan as well as Pakistan. It has been targeted repeatedly by U.S. drone strikes.

A similar event last year - that killed two Pakistani soldiers - prompted the Pakistani army to immediately close a key border crossing to NATO supplies heading from Pakistan into landlocked Afghanistan, dramatically exposing the vulnerability of the war effort.