WASHINGTON - US Defense Secretary Chuck Hagel on Tuesday hailed the NATO's transfer of security responsibility in Afghanistan as a "critical milestone" achieved by the determination of Afghan people and sacrifices by NATO and Afghan troops.

"This critical milestone is a tribute to the determination of the people of Afghanistan to take responsibility for their country's future, and it was made possible by heavy sacrifices on the part of the United States, coalition partners, and the Afghan forces," Hagel said in a statement.

"I appreciate the leadership of (Afghan) President (Hamid) Karzai in helping oversee this transition and commend NATO Secretary General (Anders Fogh) Rasmussen and Gen. Joe Dunford, the commander of the International Security Assistance Force, for their tireless efforts," Hagel said.

NATO forces handed over security responsibility to Afghan troops Tuesday to end combat missions in the country. The handover paved the way for the pullout of around 97,000 troops from 48 countries, including 66,000 Americans, by the end of 2014.

A much smaller force will be left to carry out training and advisory missions for Afghan troops after the 2014 pullout of NATO forces.

"This achievement keeps us and our coalition partners on track to bring our combat mission to a close next year and transition to a noncombat train, advise and assist mission that will help ensure Afghans can sustain security into the future," Hagel said.

In a statement released earlier Tuesday, Rasmussen said the security transfer "marks an important milestone" as NATO forces will no longer plan, execute or lead combat operations since Tuesday.

"NATO and Afghanistan, along with our partners, will continue this journey together, based on a new relationship that will remain strong for the years to come," he said.