More signs that the Bay Area’s overheated rental market is cooling off: At an industry conference Tuesday, two executives of major apartment owners said that increased supply is causing them to lose pricing power, although they think it will be temporary.

Early this year, Equity Residential was able to sign new leases in the Bay Area at 5 percent more than expiring leases, its chief operating officer David Santee said. “But literally in probably three to four weeks, as we began to enter the peak leasing season, we saw that pricing power erode from 5 percent down to 1 percent where we are today. That was kind of the impetus” behind the change in the guidance the company gave Wall Street analysts last week.

On June 1, Equity Residential said it expects revenues from properties open at least one year to grow 4 to 4.5 percent in 2016, down from its previous estimate of 4.5 to 5 percent. These revenues increased 5.1 percent last year and 4.6 percent in the first quarter versus the same quarter last year. In the press release, it blamed the revision on “continued weakness in its New York portfolio and recent underperformance in the company’s San Francisco portfolio.”

Santee attributed the Bay Area slowdown to new supply in the “urban core” and South Bay. Santee’s comments were made at the National Association of Real Estate Investment Trust Investor Forum in New York City.

On the same panel, the firm’s chief executive David Neithercut said he is still seeing strength in New York and San Francisco, “just not as much as we had expected.” He said the “headwinds” his firm is facing in San Francisco are temporary.

Based in Chicago, Equity Residential is one of the nation’s largest apartment owners. About 20 percent of its assets are in the Bay Area. According to its website, it owns 52 communities in the Bay Area. In San Francisco it has several large new luxury complexes including Potrero 1010, with 453 units, and Azure Apartments, with 273 units in Mission Bay. Both have been advertising offers such as four to six weeks free rent.

Shares in Equity Residential have tumbled 7.2 percent since last week’s announcement. Stock in other apartment real estate investment trusts also have fallen. AvalonBay is down 5.6 percent. Essex Property Trust is down almost 8 percent.

On Monday, AvalonBay reiterated its previous forecast: It expects revenues for communities open at least a year to increase 4.9 to 5.1 percent for the current quarter.

Essex, based in San Mateo, owns and operates about 60,000 units in coastal urban markets of Washington and California, CEO Michael Schall said at the same industry conference.

He said that Southern California and Seattle are running ahead of guidance this year but “Northern California has disappointed a little bit.” Year to date, revenues from Northern California properties open at least one year are growing 8.5 percent versus guidance of 9 percent.

The problem in Northern California is a “supply delivery issue,” he said. About 70 percent of the new units slated to open this year will become available in the second and third quarters. That is putting downward pressure on rents, especially in San Francisco’s South of Market neighborhood, the Peninsula and North San Jose. “Once that supply abates ... we get back into a more supply-demand equilibrium type of market and have better pricing power from that point on,” he said.

Kathleen Pender is a San Francisco Chronicle columnist. Email: kpender@sfchronicle.com Twitter: kathpender