The United States is preparing to send additional combat aircraft to the Middle East in response to threats from Iran and its proxies in Gaza, Lebanon and Yemen to attack Israel in the coming days to avenge the death of Ismail Haniyeh this week, American officials said on Friday.

One U.S. military official, speaking on the condition of anonymity to discuss operational matters, said that American forces in the Middle East were taking “necessary measures” to increase combat readiness and to protect U.S. troops and allies against any threats from Iran or Iran-backed militia groups.

How many planes to send is still being worked out, as are final approvals from senior officials including Defense Secretary Lloyd J. Austin III. Officials said they were seeking to calibrate the American response to send enough of the right types of aircraft as quickly as possible to help defend Israel without appearing to escalate the conflict.

Sabrina Singh, a Pentagon spokeswoman, raised the possibility on Friday that the United States could also send additional troops to operate whatever additional capabilities the Pentagon sends to the region. The support, she said, would be defensive in nature.

She said that during a telephone call Mr. Austin held with his Israeli counterpart, Yoav Gallant, on Friday morning, Mr. Austin “committed” that the United States would help Israel in its defense. “We will be bolstering our force protection in the region,” she said.

The Pentagon is also bracing for the possibility that Iran-backed groups, including the Houthis in Yemen and Kataib Hezbollah in Iraq, might target American troops in the region as part of the expected Iranian retaliation for the killing of Mr. Haniyeh.

Mr. Austin, during the conversation with Mr. Gallant, expressed concerns about the dangers of escalation. He said that “all countries” in the region would benefit from de-escalation, Ms. Singh said.

In addition to scores of ground-based combat aircraft, the Pentagon has already deployed more than a dozen warships in the region. The aircraft carrier Theodore Roosevelt, equipped with about 40 F/A-18 Super Hornet and F-35 attack planes, is now steaming near the Arabian Gulf, while the U.S.S. Wasp amphibious ready group, with 30 airplanes and helicopters as well as 4,500 Marines and sailors, is operating in the eastern Mediterranean Sea.

Any additional air power could be crucial. Iran fired more than 300 drones and missiles against Israel in a major attack in April, but only a handful got through, causing only slight damage. U.S. Air Force jets based in Jordan and in Saudi Arabia coordinated with French, Jordanian and British Air Force fighters to shoot down more than 80 drones.

Iran telegraphed that strike in advance, giving the Pentagon sufficient time to move additional combat aircraft and Navy ships into place while U.S. commanders negotiated access to airspace for fighter jets to operate in and coordinated air defense batteries on the ground to help defend Israel.

It’s unclear whether Israel and its allies will have that much time to prepare for any new round of major Iranian attacks, officials said.