At least 26 people were killed in a string of attacks across Iraq on Wednesday, including two U.S. troops shot dead by an Iraqi soldier.

U.S. military officials say the Iraqi soldier opened fire on U.S. troops inside an Iraqi army base in the northern city of Mosul. U.S. officials denied there was any altercation before the attack. At least six other U.S. soldiers were wounded. The Iraqi assailant was killed by return fire.

Bombings rocked Baghdad for a third straight day, continuing a wave of violence in the capital city.

Two separate car bombs exploded - one in a bustling downtown area and the other near a school in the northern Shaab neighborhood. Later, a car bomb and a roadside bomb blew up simultaneously in the eastern New Baghdad district.

An Iraqi military spokesman, Major General Qassim al-Moussawi, said in a statement Wednesday that security forces were increasing measures to prevent terrorist attacks in Baghdad.

Also, Iraqi police said gunmen killed two Christian sisters and wounded their mother outside their home in Mosul, one of the country's most religiously diverse cities.

At least 12,000 people have fled the city during a campaign of threats and attacks against the Christian community, although some have begun to return.

In a separate development, Syria's foreign minister, Walid Muallem, confirmed his country would host a U.S.-backed conference on security in Iraq on November 22. The foreign minister made the announcement after meeting with Iraqi Foreign Minister Hoshyar Zebari in the Syrian capital, Damascus.

It is Zebari's first visit to Syria since a raid on a Syrian village last month that Damascus says was carried out by U.S. forces based in Iraq. U.S. officials say the troops were targeting insurgents, but Syrian officials say eight civilians died in the cross-border attack.

During his visit, the Iraqi foreign minister said Iraq will not be used as a base for attacks against neighboring countries. He also stressed the need for security coordination in his country, after meeting with Syria's president Bashar Assad on Wednesday.

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