An Israeli official said did not specify what part of the hospital troops were searching.
Israeli troops found weapons and "terror infrastructure" during an on-going raid within Al Shifa hospital in the Gaza Strip, a senior military official said on Wednesday.
He did not spell out what had been found or provide any visual evidence, but said this would be presented later.
The official, who declined to be named, told reporters that there had been no fighting inside the hospital complex after the soldiers arrived during the night, and that there was no friction with medical staff or patients.
He did not say where the soldiers were operating, but indicated they were in a different section of the site to where medics and patients were located.
"IDF soldiers have already found weapons and other terror infrastructure. In the last hour, we saw concrete evidence that Hamas terrorists used the Shifa hospital as a terror headquarter," the official said.
Hamas denied this.
"(This claim) is nothing but a continuation of the lies and cheap propaganda, through which (Israel) is trying to give justification for its crime aimed at destroying the health sector in Gaza," Hamas said in a statement.
The Israeli official said soldiers had undertaken "an extremely precise and targeted operation" and did not say how long it would last. "Our soldiers are making a slow and deliberate progress based on our intelligence," he said.
The Israelis have said for years that Hamas militants have used Al Shifa, the largest hospital in Gaza, as a cover for their operations. Hamas and hospital staff have repeatedly denied this.
The military official said four militants had died in a gunfight outside Al Shifa, as Israeli soldiers sought to enter the complex. He added that soldiers had interrogated individuals found in the area they were searching.
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