Iraqi police say gunmen have killed six policemen and wounded several others in an attack on a police checkpoint in the northern part of the country.

Officials say the ambush took place early Wednesday in the Gayara region, south of Mosul.

In other news, police say robbers in camouflage uniforms stole Iraqi currency worth $550,000 from a vehicle traveling in Baghdad.

Also Wednesday, the U.S. military said Iraqi and U.S. forces detained 28 suspected terrorists in operations targeting al-Qaida in Iraq terrorist cells. Two of the detainees were believed to be planning an attack on the Mosul airfield during Ramadan.

Iraq's National Security Adviser Mowaffak al-Rubaie said Wednesday he expects the number of U.S. troops in Iraq to drop to about 100,000 by the end of next year.

The top U.S. commander in Iraq, General David Petraeus, said an improved security situation in Iraq could allow 30,000 troops to be withdrawn by mid-2008.

Washington boosted U.S. troop levels by about 30,000 in a surge that began early this year, in addition to the around 130,000 U.S. troops already in Iraq.

Also Wednesday, U.S. Secretary of State Condoleezza Rice told NBC television that the United States will ensure Iraq's security from what she called "troublesome neighbors" such as Iran.

Iranian President Mahmoud Ahmadinejad has said that Iran would fill a vacuum if U.S. troops withdrew from Iraq.

Some information for this report was provided by AFP, AP and Reuters.