The top U.S. military officer said Tuesday that he's confident that most of the 30,000 additional troops that are being sent to Afghanistan will be there by August.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, told reporters traveling with him in Afghanistan that the first 16,000 troops who already have orders will be in on schedule.

On Monday, Lt. Gen. David Rodriguez, the second-highest ranking U.S. general in Afghanistan, said the rapid escalation of American troops would take longer than expected, possibly as long as 11 months. Rodriguez blamed the delay on the logistical challenges the military faces in bringing in so many forces so quickly.

But Mullen said that he's "reasonably confident" the logistics can be made to work, although "I want a plan B because life doesn't always work out."

He said the vast majority of troops in the surge ordered by President Barack Obama should be in Afghanistan by August.

Mullen on Tuesday toured U.S. bases in eastern Afghanistan, as well as a small village, where he met an Afghan village elder and the local governor. He said such visits "make me continue to be aware of the gap" between strategic plans and ground-level reality.

Later, Mullen flew to Islamabad for meetings with Pakistan's military leaders, including military chief Gen. Ashfaq Parvez Kayani. He was also scheduled to speak to students at Pakistan's National Defense University.

Earlier, Mullen said the Taliban-led insurgency in Afghanistan is harder to defeat now than it was a year ago, and said he will take up concerns about strengthening ties to al-Qaida with government leaders in Afghanistan and Pakistan.

"I remain deeply concerned by the growing level of collusion between the Afghan Taliban and al-Qaida and other extremist groups taking refuge across the border in Pakistan," Mullen said at the start of his visit Monday.

"Getting at this network, which is now more entrenched, will be a far more difficult task than it was just one year ago," Mullen said in the Afghan capital.

"As part of this trip, I intend to discuss with Afghan and Pakistani leaders the extent to which we all can better cooperate and coordinate our activities to eliminate the safe havens from which these groups plan and operate."

He said he was headed to Islamabad and will have another meeting, his 14th, with Gen. Ashfaq Parvez Kayani as well as other top Pakistani officials.

Painting a grim picture, Mullen said Afghan insurgents were dominant in one-third of Afghanistan's 34 provinces and "the insurgency has grown more violent, more pervasive and more sophisticated."