A series of bomb blasts across Baghdad killed six people and injured more than 20 others Thursday, police said, in the fourth consecutive day of heightened violence in the Iraqi capital.

Meanwhile, Iraqi officials said the U.S. has officially responded to Iraqi proposals for changes in a draft security pact that would keep American troops in the country three more years — but did not say what was in the response.

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 high point and just before the U.S. troop surge that helped bring down violence in the capital.

But 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. Police said the convoy was carrying city workers. The police spoke on condition of anonymity because they were not supposed to release information.

The twin blasts in the capital's Sunni enclave of Sheik Omar happened at a checkpoint manned by members of an Awakening Council, the mostly Sunni groups that have joined forces with the Americans against al-Qaida in Iraq.

Another two Awakening Council members was killed in a bombing just before noon in southeastern Baghdad. The councils come under frequent attacks by insurgents because they have sided with U.S. forces.

Roadside bombs targeting two separate convoys carrying Baghdad city officials injured eight people, the mayor's office said in a statement. The municipal officials were not hurt in the attacks.

Nine other people were wounded in a roadside bomb attack in Baghdad's sprawling Shiite slum of Sadr City, police said.

Violence has dropped sharply in Baghdad since the Sunni revolt — led by Awakening Councils — against al-Qaida and the routing of Shiite militias in Baghdad and southern Iraq last spring.

The Iraqi Cabinet asked on Oct. 21 for changes to the draft security pact being negotiated with Washington, including a demand for expanded Iraqi legal authority over U.S. soldiers, which the U.S. has described as a "red line."

Other changes would rule out the use of Iraqi territory to launch attacks against neighboring countries, effectively rule out any extension of the U.S. military presence beyond the end of 2011 and allow Iraqis to inspect U.S. military shipments in and out of Iraq.

Iraqi lawmakers have said the changes are essential if parliament is to approve the agreement by a year's end deadline.

Government spokesman Ali al-Dabbagh told The Associated Press on Thursday that the U.S. has responded to the proposed changes. He gave no further details and declined to characterize the U.S. reply.

U.S. Embassy spokeswoman Susan Ziadeh confirmed the report but did not give details.

But another top Iraqi official said the U.S. accepted some proposals and rejected others, presumably an Iraqi demand for expanded legal control over U.S. soldiers. The official would not elaborate and spoke on condition of anonymity because he was not supposed to discuss the issue publicly.

Another official said the written response was sent to Prime Minister Nouri al-Maliki who was studying it. The official spoke on condition of anonymity for the same reason.

Iraq officials have said U.S. diplomats appeared willing to make the changes except for expanded jurisdiction. The Iraqis had urged the U.S. to show flexibility on that issue, which would open the door to limited prosecution by Iraqi courts of major crimes committed by soldiers off post and off duty.

Without an agreement or a new mandate, the U.S. would have to suspend all military operations in Iraq.

Also Thursday, Romania announced it plans to withdraw the country's 500 peacekeeping troops from Iraq by the end of the year. Some Romanian military personnel will remain to work as counselors.