In recent days, fighting near the Al-Shifa hospital in northern Gaza has intensified and supplies have run out.
Israeli authorities claim that Hamas has established command posts underneath the hospital and other health facilities. Hamas and Al-Shifa staff deny the existence of the tunnels.
Al-Shifa hospital director Mohammed Abu Selmia was cited by the Associated Press as saying that the facility lost power on Saturday.
Spokesmen for the Health Ministry in the Hamas-run Gaza Strip said six patients died at the hospital after the generator was shut down.
Israel's military confirmed clashes outside the hospital, but army spokesman Daniel Hagari denied Al-Shifa was under siege.
He said troops would on Sunday assist in moving babies treated there and said "we are speaking directly and regularly" with hospital staff.
