The head of the Transportation Security Administration warned travelers Friday to expect long airport security lines to continue during the peak summer travel season despite Congress' shifting of $34 million to the agency.

Peter Neffenger briefed officials in Chicago on Friday about efforts to address crushing delays in getting passengers through security checkpoints at major airports around the country. Congress agreed to shift forward the $34 million in TSA funding to let the agency pay overtime to existing staff and hire an extra 768 screeners by June 15. The agency is funneling many of those resources to major hub airports like Chicago's O'Hare International.

Neffenger called the money a "good down payment" but said more resources were needed to address a shortage of screeners.

"I think this summer is going to continue to be a challenge," he said at a news conference at O'Hare after meeting with members of Illinois' congressional delegation and Chicago Mayor Rahm Emanuel. "I think we're doing everything we can to mitigate that from the larger standpoints. I think you'll still see crowds in airports. My goal is to keep you moving. We can't have a situation like we had here in Chicago."

Despite Neffenger's call for more resources, there is no grand plan to return staffing to former levels, and the agency is suffering attrition rates of as high as 28 percent among its part-time workers, he said.

U.S. Sen. Dick Durbin acknowledged some of the things Congress has done to cap the number of screeners and limit resources were unfair, but he said TSA has a responsibility to manage its limited resources effectively.

Emanuel told reporters the delays were unacceptable and especially aggravating since there were signs weeks ago of a looming security meltdown. He said Chicago would monitor TSA's progress with an "accountability scorecard."

More people are flying, thanks to a mix of a strong economy, more flights and lower fares. At the same time, TSA and Congress cut the number of screeners by 10 percent on expectations that an expedited screening program called PreCheck would speed up the lines. However, not enough people enrolled. To reduce lines, TSA had also been randomly placing passengers from the regular screening lanes into the faster PreCheck lanes in a program called Managed Inclusion but stopped in the fall after government auditors found lapses in security.

Neffenger ruled out any return to Managed Inclusion, but he said there's been a "tremendous" increase in the number of PreCheck enrollments in recent weeks.

Chicago's two airports are getting more bomb-sniffing dogs and an extra 58 screeners immediately, with nearly 250 more by mid-August. One hundred other staffers will shift from part-time to full-time employment.