The top two U.S. defense officials called on Pakistan Wednesday to improve security along its border with Afghanistan, in order to stop the flow of insurgents.  VOA's Al Pessin reports from the Pentagon.

The Chairman of Joint Chiefs of Staff, Admiral Mike Mullen, says he told Pakistani leaders during a visit to Islamabad last week that the porous border is a key factor in the increase of insurgent attacks in Afghanistan, and that Pakistan must take steps to improve the situation.

"The border there is a really critical issue that we're going to have to solve," said Admiral Mullen. "Certainly that's a message that I delivered to each of the leaders that I visited in Pakistan.  And it has to be solved sooner rather than later.  The bottom line is this, we're seeing a greater number of insurgents and foreign fighters flowing across the border with Pakistan unmolested and unhindered.  This movement needs to stop."

Admiral Mullen said insurgents use safe havens in Pakistan to train for what he called "bolder, more sophisticated" operations, like the attack on a U.S.-Afghan outpost last weekend that killed nine U.S. troops.  But he denied Pakistani news reports that he gave an ultimatum to Pakistani leaders during his meetings last week that if Pakistan did not take action along the border the United States would.

Speaking with the admiral at a news conference Wednesday, Defense Secretary Robert Gates denied reports of a U.S. buildup on the Afghan side of the border in preparation for an incursion.  But he declined to rule out U.S. action in the future.

"We will take defense actions," said Secretary Gates. "We have taken defensive actions when fired upon from places right across the border.  Generally that's been counter-artillery.  And beyond that, I think I won't say."

Secretary Gates also reinforced Admiral Mullen's view on the importance of getting control of the tribal areas on Pakistan's side of the border.

"There is no question that the absence of pressure on the Pakistani side of the border is creating an opportunity for more people to cross the border and to launch attacks," he said. "There are efforts under way to try and improve that on both the Pakistani side and on the Afghan and coalition side in Afghanistan.  But I think clearly, as the admiral said earlier, there is a real need to do something on the Pakistani side of the border to bring pressure to bear on the Taliban and some of these other violent groups."

Secretary Gates said Pakistani leaders need to gain what he called "a full appreciation of the magnitude and reality of the danger" the militants pose to their own country, as well as to Afghanistan and the coalition forces there, and the importance of getting control of the border area.  He also said Pakistani forces have launched some operations along the border during the last few weeks, but he wants that effort to "intensify."

Both the secretary and the admiral said they are working to identify additional U.S. combat units that can be sent to Afghanistan to help deal with the resurgent Taliban and other violent groups.  But the admiral and other officials have said any significant U.S. troop increase in Afghanistan will probably have to wait until next year, under a new U.S. president, because so many troops have been deployed to Iraq, or are now in their year-long rest and retraining periods.