PORTSMOUTH, N.H. — A transportation official says a lift bridge connecting New Hampshire and Maine that was out of commission for a few days last month is deteriorating at a steady pace.

Douglas Gosling, director of bridge maintenance for the New Hampshire Department of Transportation, told the Portsmouth Herald the Sarah Mildred Long Bridge should last until its replacement is completed in 2017. But he said he can’t guarantee that.

The 73-year-old bridge was shut down last month after its center span got stuck during a routine test.

Gosling said the center span isn’t the biggest problem — it’s the rusting floor beams and supports.

The bridge carries the Route 1 Bypass over the Piscataqua River between Portsmouth and Kittery.

In 2010, the governors of New Hampshire and Maine recognized the need to upgrade the bridge. Renovations are planned, but the funding is not entirely in place yet. New Hampshire officials say it is the state’s top red-listed bridge.

The bridge, as well as the Interstate 95 bridge over the Piscataqua, have been getting more traffic as workers build a replacement Memorial Bridge, the third one connecting Kittery and Portsmouth. The Memorial Bridge is expected to open in the summer.

On the Memorial Bridge, inspections and subsequent work became more frequent in the last several years of its life. The posted weight limit was reduced until it was at 5 tons, before it was closed to traffic. It was nearly 90 years old when work began to replace it.

The Long Bridge is currently posted at 20 tons. Gosling said he does not see any immediate downgrade.

However, he said, “we’re real close” to the beginning of the same kind of rapid deterioration that plagued the Memorial Bridge in its last years.

“It becomes exponential, if you will. It picks up speed,” Gosling said. “We’re trying to hold everything together. We’re doing the best we can.”