Afghan officials say seven police officers have been killed in southern Afghanistan, as NATO troops begin to hand over security control of some areas to Afghan forces.

Authorities said the officers were killed Monday at a checkpoint near Lashkar Gar in Helmand province.

The killings came as U.S. General David Petraeus ended his command of American and NATO troops in Afghanistan.

U.S. General John Allen became the top U.S. commander during a ceremony in the Afghan capital, Kabul.

Allen, who headed the U.S. Central Command, said it is his intention to maintain the “momentum of the campaign.” He said he wants to see Afghanistan become a secure and stable environment free from extremism and terrorism.

Petraeus said in his farewell address that progress has been made in Afghanistan, but also warned of a tough fight ahead.

He commanded 140,000 U.S. and coalition forces Petraeus is leaving the post to head the U.S. Central Intelligence Agency.

Petraeus received a medal from Afghan President Hamid Karzai and left the country after the ceremony. On the way home to Washington, the general paid a farewell visit to Turkish Chief of General Staff General Isik Kosaner in Ankara.

The change of command comes at a time when international forces have begun to pull out of the war-torn country, and violence has reached its worst levels in the almost 10-year war.

In other violence Monday, NATO said separate bomb attacks in the country's south and east killed four troops, while another service member died as a result of a non-battle related injury in the north.

A roadside bomb killed the police chief of Registan district and three other police officers in southern Kandahar province, and at least two Afghans were beheaded in western Farah province after being abducted last week along with at least 30 others for supporting the Afghan government. More than a dozen of the kidnapping victims have been released.

On Sunday, Bamiyan province became the first of seven areas to be handed over during the first phase of the transition, which is expected to be completed by the end of 2014. The United States and allied countries are set to withdraw all combat troops from Afghanistan in three years.

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.

During Monday's transfer of command ceremony in Kabul, Afghan Defense Minister Abdul Rahim Wardak warned against a hasty drawdown of foreign forces.