Negotiations to free two corrections officers held hostage in a prison tower for four days were briefly disrupted Wednesday when one of the inmate captors set off a gas canister that landed in a prison yard.

No one was injured, and prison officials did not retaliate, said Cam Hunter, a spokeswoman for the Arizona Department of Corrections (search). She said it wasn't clear what provoked the inmate, and added that negotiations soon resumed.

"The dialogue with the inmates is continuing," Hunter said. "They're [negotiators] going to ride this thing out as long as it takes."

The standoff began Sunday morning after an inmate attacked a guard in the kitchen of the medium- to high-security Arizona State Prison Complex-Lewis (search). The prisoner and another inmate then got into an observation tower where the two guards, one male and one female, were stationed.

Negotiators got their first glimpse of the guards on Tuesday, and they appeared to be OK, Hunter said. The guards were allowed to speak briefly to law enforcement by radio.

Prison officials had initially said the male guard was injured, though they did not disclose details of the injury. The identities of the inmates and guards have not been released.

The guard tower, in the middle of the prison grounds, is believed to be stocked with weapons. Authorities would not say whether the inmates had made any threats.

Until the crisis is resolved, the 4,400 inmates at the prison west of Phoenix remain in lockdown, meaning no one is allowed in the yards and inmate movement is more heavily restricted.

Hunter said the restricted airspace around the prison has been increased, raising the minimum altitude over the area from 3,000 feet to 5,000 feet. Flight restrictions, which extend three miles around the prison, were put in place when the standoff began.

The restrictions were increased partly to avoid alarming the inmates, said Ivan Bartos, the warden at the state prison in Yuma, who has been assisting in the standoff. "Some of the most innocent things could derail something that's taken hours to build," he said.