New research by jobs website Indeed found it is taking slightly longer to fill jobs in the US and positions still empty after 30 days can take months to fill.

Indeed found the percentage of jobs unfilled after 30 days rose to 57.0% in the three months ended October 2014 — up from 56.2% in the same period in 2013.

Paul D’Arcy, senior VP at Indeed, said the strengthening US labor market is behind the increase. As more employers compete for workers with the top skills, it becomes harder and takes longer to fill jobs.

“Demand now for skilled labor is very, very high and rapidly strengthening,” D’Arcy said. “IT, software engineering and healthcare are kind of the key areas where there is a tremendous skills shortage nationally.”

Another key finding by Indeed: 30 days proved a tipping point. Positions not filled within that time frame have a 57% chance of remaining open for three months or more. While employers may still be confident after 30 days, the research shows they should look at their strategy or the time to fill could last months more.

Indeed’s report also showed variance by industry and geographic area.

The economy is getting better and interesting in many ways, according to Indeed’s D’Arcy. The US is becoming two separate economies as companies fight to hire workers with the most in-demand skills while individuals without those skills can still have difficulty finding jobs.

The report found Western states such as Montana and Oregon are able to fill positions more quickly while Eastern and Midwestern states generally display longer timelines.

Indeed listed the 20 most difficult-to-fill job openings in the 10 largest US cities in December 2014:

The Indeed report looked at all types of jobs, not just temporary agency positions. A survey of staffing firms last year by Staffing Industry Analysts found median time to fill for temporary staffing positions was five days. The time-to-fill varied by segment with median time to fill at industrial staffing firms being two days and median time to fill for IT staffing firms being 10 days.

On the other hand, time to fill took longer for direct hire (permanent placement), according to the research. The median time to fill for direct hire positions was 24 days.

Time-to-fill can be a closely watched ratio for contingent workforce managers. A 2013 survey of staffing buyers found it among the most cited methods used to evaluate managed service providers.