BAGHDAD, Iraq - Gunmen ambushed a bus carrying Iraqi workers to a U.S. air base near Baqouba, killing 13, while Iraq's president predicted today that a new constitution could be hammered out by the end of the month.

Assailants in two cars attacked the minibus on its way to al-Faris air base, said Police Col. Mahdi Saleh from Khalis. Baqouba is 35 miles northeast of Baghdad.

Another police official, who declined to give his name, said 10 workers were killed, along with three civilians, when the bus slammed into a nearby car.

In Baghdad, President Jalal Talabani, a Kurd, said good progress was being made on drafting a new constitution, but that there are some "Arab brothers" who "have some reservations that are being taken into consideration."

"If we can reach an agreement with them, I believe the constitution can be ready by the end of the month," he said.

Talabani did not identify the critics but he was clearly referring to Sunni Arabs who want a strong statement affirming Iraq's Arab identity and who oppose some definitions of federalism supported by the Kurds and some Shiite Muslims.

The Iraqi parliament has until Aug. 15 to adopt a draft constitution, which will be put to a nationwide referendum by mid-October. If approved, it will provide the basis for a new election in December.

Former Prime Minister Ayad Allawi, a secular Shiite, said he supported using the interim constitution, approved before Iraq regained sovereignty from the U.S.-led occupation authority.

Some Shiites oppose the interim charter's limitations on the role of Muslim law as the basis of the legal code. Some Sunnis oppose autonomy guarantees given to the Kurds as a threat to national unity.

"I believe that will allow Iraq to have the constitution. As soon as the referendum on it is finished, Iraq will have stability," he said.

Violence has steadily escalated in Iraq since Prime Minister Ibrahim al-Jaafari announced his Shiite-dominated government in April. Most of the insurgents are Sunni Arabs.

On Monday, gunmen killed at least 24 police, soldiers and government workers in Iraq in a series of ambushes and shootings.

A new security offensive in Baghdad has captured about 50 suspected insurgents, including two Syrians, in its first few days, an Iraqi general said.