U.S. General David Petraeus has handed over command of American and NATO troops in Afghanistan to U.S. General John Allen, transferring responsibility at a time when international forces are beginning to pull out of the war-torn country.

Petraeus steps down after a mixed year in which he oversaw a surge of 30,000 additional U.S. troops aimed at reversing the Taliban insurgency, particularly in southern Afghanistan. He commanded some 140,000 U.S. and coalition forces at a time when violence reached its worst levels in the 10-year war.

General Allen officially took command at a ceremony Monday in the Afghan capital, Kabul. Petraeus is leaving the post to head the U.S. Central Intelligence Agency

In his farewell address, Petraeus warned of a tough fight ahead, noting “there is nothing easy about such a fight especially when the enemy can exploit sanctuaries outside the country.” The military has accused neighboring Pakistan of not doing enough to fight Taliban and al-Qaida-linked militants in the country's tribal region along the Afghan border.

Allen, who headed U.S. Central Command, also warned of “tough days ahead” but said it was his intention to maintain the “momentum of the campaign.” The new commander of U.S. and NATO forces said he wants to see Afghanistan become a secure and stable environment free from the extremism and terrorism that has plagued the country.

Violence has surged in Afghanistan. On Monday, NATO said a bomb attack killed three of its service members in the country's east.

The day before, a senior advisor to Afghan President Hamid Karzai and a member of parliament were killed in an attack in Kabul.

The Taliban claimed responsibility for the killing of Jan Mohammad Khan, the former governor of Uruzgan province. Khan was killed along with Uruzgan lawmaker Mohammad Ashim Watanwal late Sunday when two men wearing suicide vests targeted Khan's home in western Kabul's Karte Char district.

The attack came as Bamiyan province became the first of seven areas to transfer security responsibility from international forces to Afghan troops. A ceremony was held Sunday in the province to mark the handover from New Zealand troops to local forces.

The security transition is expected to be completed by the end of 2014, as the United States and allied countries withdraw combat troops from Afghanistan.

Last month, U.S. President Barack Obama announced he will withdraw 33,000 of the 100,000 American troops in Afghanistan over the next 14 months.