The United Arab Emirates and Egypt have carried out a series of airstrikes in the Libyan capital, Tripoli, U.S. officials said Monday, marking an escalation in the chaotic war among Libya’s rival militias that has driven American and other diplomats from the country.

The Obama administration did not know ahead of time about the highly unusual military intervention, although the United States was aware that action by Arab states might come as the crisis in Libya worsened, said one official who spoke on the condition of anonymity because of the sensitivity of the matter.

The airstrikes appear tied to fear over the growing muscle of Islamist militias. The region’s monarchies and secular dictatorships are increasingly alarmed about Islamist gains from Libya to Syria and Iraq. And the airstrikes may signal a new willingness by some Arab states to take on a more direct military role in the region’s conflicts…

U.S. officials have long been worried about a proxy war in Libya, with Saudi Arabia, Egypt and the UAE backing more secular militias against Islamist forces supported by Qatar. The United States says Qatar is arming and funding Islamists in Libya.