BAGHDAD (AP) — A series of bomb blasts across Baghdad killed four people and injured more than 20 others today, police said, in the fourth consecutive day of heightened violence in the Iraqi capital.

U.S. officials say attacks in Baghdad are averaging about four a day — down nearly 90 percent from levels in late 2006, when Shiite-Sunni fighting was at its highest and just before the U.S. troop surge that helped bring down violence in the capital.

There has been a marked uptick this week, with a string of daily bombings in the capital that has killed more than 30 people and wounded around 80 others since Monday.

The violence shows that insurgents remain a threat, even in the heavily secured Iraqi capital.

The deadliest attack Thursday came near a checkpoint in central Baghdad when two bombs exploded during the morning rush hour, police said. Four people were killed and seven wounded in the blasts.

Another bomb targeting a government convoy injured six people, police and hospital officials said.