Both attacks occurred in Sunni Arab areas where al Qaeda in Iraq has been active. They appeared to be part of a campaign by both Sunni and Shiite extremists to undermine U.S. efforts to shore up local administrations and secure the security gains achieved since early last year.
The target of the Mosul blast appeared to be the provincial governor, who was near the explosion but escaped injury.
Col. Fawzi Fraih, civil defense director of Anbar province, said dozens of sheiks had gathered in a building in Karmah, 20 miles west of Baghdad, for a meeting attended by U.S. officials when the bomber struck.
Local police Capt. Amir al-Jumaili said 20 Iraqis were killed and 20 others wounded.
The U.S. command said two interpreters were killed along with three Marines assigned to Multinational Forces-West. It was unclear if the interpreters were among the 20 dead reported by the Iraqis.
U.S. authorities suspected al Qaeda in Iraq was behind the attack.
Two policemen said the bomber was able to penetrate security because he was a wearing camouflage uniform of the Iraqi police commandos. Both policemen spoke on condition of anonymity for security reasons.
The blast occurred only days before U.S. troops are to hand over security responsibility for Anbar to the Iraqis, marking a major milestone in the campaign to lower the U.S. profile in an area that had once been center-stage of the war.
Anbar sheiks spearheaded the Sunni revolt against al Qaeda, one of the key reasons behind the dramatic drop in both overall violence and American casualties since 2006.
The media office for Anbar province said the dead included the town's administrative director and at least two chiefs of major Sunni tribes in the area.
The bomb in Mosul went off between the government headquarters and a market, where the governor of surrounding Nineveh province, Duraid Kashmola, was inspecting damage from an earlier rocket attack, police said.
U.S. authorities said 18 people were killed and nearly 80 wounded - mostly civilians. Mosul is the scene of an ongoing Iraqi military operation against al Qaeda and other Sunni extremist groups.
The street where the blast occurred had been blocked with concrete barriers but was reopened about three weeks ago as part of a government move to improve the quality of life and undermine support for extremists.
Adil Khalid, a 35-year-old grocer, said he went to the market to buy food from wholesalers when he saw a parked car explode about 100 yards (meters) away.
"It was like doomsday. People were panicked, running to escape," Khalid said. "Even policemen fled the scene but came back to evacuate the victims. I saw two or three bodies burned beyond recognition."
The two bombings were part of an uptick in violence that has pushed the monthly death toll for U.S. troops in Iraq to at least 26. That's well below figures of last year but an increase over the 19 who died in May, the lowest monthly tally of the war.
In all, at least 4,110 U.S. military service members have died in the Iraq war since it began in March 2003, according to an Associated Press count.
Ten people, including four Americans, were killed Tuesday in a bombing in a municipal council office in the Shiite area of Sadr City in Baghdad.
Two Americans were shot dead and four wounded Monday when a disgruntled official opened fire as they left a municipal building in Salman Pak about 15 miles south of the capital.
In a Web statement posted Thursday, the al Qaeda front group the Islamic State of Iraq claimed responsibility for a bombing that killed three American soldiers and their interpreter in Nineveh two days ago.
The statement said the attack was in retaliation for the killing of a Muslim family in Mosul.
The U.S. military says violence in Iraq has dropped to its lowest level in more than four years, but attacks are continuing as Sunni and Shiite extremists try to regroup and undermine security gains.
In other developments: