"I am little concerned about United Technologies and Raytheon," Trump said on CNBC, wondering whether the combination will "take away more competition."

The companies announced on Sunday that they will merge, creating a behemoth American aeronautics and defense company called Raytheon Technologies Corporation.

"When I hear they're merging, does that take away more competition? It becomes one big fat beautiful company," he added.

"But I have to negotiate, meaning the United States has to buy things and does that make it less competitive? Because it's already non-competitive."

In making their announcement, the companies said they did not expect US antitrust authorities would block the deal, given that very little of their business operations overlapped.

The UTC merger with Raytheon would transform the two companies into a single conglomerate with varied but well-established brands, each in the top tier of its specialty.

Raytheon is best known for its Patriot air defense systems, which gained fame during the first Gulf War, and its Tomahawk cruise missiles, often the first weapons fired from US navy ships in recent conflicts.

UTC is a big player in the aeronautics industry with its Pratt and Whitney engines, which are used in civil and military aircraft, including the F-35 multi-role stealth fighter, considered one of the most advanced combat aircraft in the world.