Mullen added that he had told troops headed for Afghanistan to "steel themselves for more combat and more casualties".
"The insurgency has grown more violent, more pervasive, more sophisticated," he said.
Al Jazeera's Todd Baer reports on concerns over an increasingly tough Afghan fight
Earlier this month Barack Obama, the US president, ordered an extra 30,000 troops to Afghanistan as part of a revised strategy to turn the war around and eventually pave the way for a US pullout.
Mullen's comments came as a senior US commander in Afghanistan warned that the extra troop deployment would likely take longer than expected.
Lt. General David Rodriguez said logistical challenges meant it would probably take nine to 11 months to deploy all the new troops to the region.
Obama's original plan had envisaged the additional deployment being in place within six months.
Underscoring the fragile security situation in Afghanistan, two separate attacks left at least 16 Afghan policemen dead on Monday.
The attacks on checkpoints in Helmand and Baghlan provinces again highlighted the vulnerability of Afghan police, whose training, along with that of the local military, is a priority in the recently announced US strategy for Afghanistan.
The initial vanguard of the 30,000 extra US troops – a 1,500-strong force of Marines - is expected to land in Afghanistan later this week.
They will be deployed in southern Helmand province, one of the toughest battlefields, to prepare the logistics for thousands more troops due in the coming months, Mullen said.
|Obama has ordered another 30,000 US troops to Afghanistan [AFP]
Mullen's added that he would address the issue of Pakistan-based fighters during talks in Islamabad later this week.
Washington has been trying to press the Pakistani government to step up its crackdown on Taliban and al-Qaeda fighters who have long used their country as a refuge.
US commanders have identified the Pakistan-based Haqqani network, widely- believed to have ties with Pakistan's spy agency, as one of the biggest threats to US forces in Afghanistan.
But Pakistani officials have been reluctant to bow to US pressure, saying their priority is eliminating the Pakistani Taliban who have been linked to a series of deadly bomb attacks across the country, rather than tackling the Haqqani network.
On Monday, in a strongly worded editorial published in the New York Times, Asif Ali Zardari, the Pakistani president, blamed the US for the situation in Afghanistan.
"When the Soviets were defeated and left in 1989, the US abandoned Pakistan and created a vacuum in Afghanistan, resulting in the current horror," he wrote.
"And then after 9/11, the United States closed its eyes to the abuses of the dictatorship of President Musharraf. For Pakistanis, it is a bitter memory."