SINGAPORE (Reuters) - The number of major U.S. defence contractors has shrunk to the point of becoming a national security concern, according to the U.S. Air Force's acquisition head, who said his service needed to have more frequent competitions to benefit smaller companies.

The comment came after U.S. President Donald Trump this month expressed concern that United Technologies Corp's plan to combine its aerospace business with that of Raytheon Co could harm competition and make it more difficult to negotiate defence contracts. The $121 billion deal would be the sector's biggest-ever merger.