Retiring Navy Admiral Mike Mullen, leaving his post as chairman of the U.S. Joint Chiefs of Staff, has said he believes there is no solution to the conflict in Afghanistan without Pakistan.

Mullen spoke at a ceremony outside Washington where his successor, U.S. Army General Martin Dempsey, was sworn in as the nation's top military officer.

Mullen said he believes the security transition in Afghanistan will be the biggest challenge for Dempsey in the coming years. U.S. troops are scheduled to turn over security responsibilities to Afghan forces by 2014.

The U.S. is facing strained relations with Pakistan over Islamabad's apparent failure to dismantle Islamic extremist groups. Mullen recently described the Haqqani terrorist network as a “veritable arm” of Pakistan's intelligence agency.

Dempsey, however, has said the biggest challenge going forward is the U.S. national debt and the security threat it poses to the United States.

In remarks at the event Friday, U.S. President Barack Obama described General Dempsey as one of the military's most battle-tested officers.

Dempsey's 37-year career includes two tours of duty in Iraq, and serving as acting commander of all military operations in the Middle East.

He moves into his new role as the military faces deep budget cuts to help trim the massive U.S. debt. President Obama has asked the Defense Department to cut $400 billion from its budget over the next 12 years. During his confirmation hearing, Dempsey warned lawmakers that cuts of $800 billion or more would be “extraordinarily difficult and very high risk.”