AMATRICE, Italy — Rescue workers acknowledged Friday they might not find any more survivors from Italy’s earthquake as they confronted a new obstacle to their recovery work: a powerful aftershock that damaged two key access bridges to hard-hit Amatrice, threatening to isolate it.

Mayor Sergio Pirozzi warned that if new roads weren’t quickly cleared to bypass the damaged ones, Amatrice risked being cut off at a time it needs as many transport options as possible to bring emergency crews in and some of the 281 dead out.

“With the aftershocks yesterday but especially this morning the situation has worsened considerably,” Pirozzi told reporters. “We have to make sure Amatrice does not become isolated, or risk further help being unable to get through.”

The biggest aftershock struck at 6:28 a.m., one of the more than 1,000 that have hit the area since Wednesday’s quake. The U.S. Geological Service said it had a magnitude of 4.7, while the Italian geophysics institute measured it at 4.8.

It left one key access bridge to Amatrice unusable, and damaged another one. Crews began clearing trees to create an alternate bypass road to avoid the nearly 25-mile detour up and down mountain roads that they were forced to use Friday, slowing the rescue effort.

Even before the roads were shut down, traffic into and out of Amatrice was horribly congested with emergency vehicles and dump trucks carrying tons of concrete, rocks and metal down the single-lane roads.

Multiple ambulances were also bringing the dead to an airport hangar in the provincial capital of Rieti, where four big white refrigerated trucks created a makeshift morgue to which relatives came in a steady stream Friday.

Premier Matteo Renzi declared a state of emergency and authorized $56 million for immediate quake relief.

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