CAIRO — Twice in the last seven days, Egypt and the United Arab Emirates have secretly launched airstrikes against Islamist-allied militias battling for control of Tripoli, Libya, four senior American officials said, in a major escalation of a regional power struggle set off by Arab Spring revolts.

The United States, the officials said, was caught by surprise: Egypt and the Emirates, both close allies and military partners, acted without informing Washington, leaving the Obama administration on the sidelines. Egyptian officials explicitly denied to American diplomats that their military played any role in the operation, the officials said, in what appeared a new blow to already strained relations between Washington and Cairo.

The strikes in Tripoli are another salvo in a power struggle defined by Arab autocrats battling Islamist movements seeking to overturn the old order. Since the military ouster of the Islamist president in Egypt last year, the new government and its backers in Saudi Arabia and the United Arab Emirates have launched a campaign across the region — in the news media, in politics and diplomacy, and by arming local proxies — to roll back what they see as an existential threat to their authority posed by Islamist groups like the Muslim Brotherhood.

Arrayed against them and backing the Islamists are the rival states of Turkey and Qatar.

American officials said the Egyptians and the Emiratis had teamed up against an Islamist target inside Libya at least once before. In recent months, the officials said, teams of “special forces” operating out of Egypt but possibly composed primarily of Emiratis had also successfully destroyed an Islamist camp near the eastern Libyan city of Derna, an extremist stronghold.