CAIRO — Unidentified warplanes on Monday bombed a small arms depot and other locations in the Libyan capital, Tripoli, that are controlled by Islamist-aligned militias, suggesting that a foreign state had intervened in the escalating battle for control of the city.

At least six people were killed, The Associated Press reported. The origin of the planes remained a mystery.

The airstrikes were beyond the capacity of the limited Libyan Air Force, and Libyan authorities said the planes had come from a foreign state. The United States, France, Italy and Egypt all denied responsibility.

“The United States was not involved whatsoever in these events,” said Marie Harf, a State Department spokeswoman.