CHICAGO, May 12 (Reuters) - United Airlines, Delta Air Lines and American Airlines have canceled flights between the United States and Tel Aviv, according to data by flight tracker FlightAware, amid an escalating conflict in Israel.

Israel said it killed 16 members of the Hamas armed wing in Gaza in a barrage on Wednesday, and Palestinian militants rained rockets into Israel in one of their most intense hostilities in years.

United Airlines said it had canceled flights from Chicago and Newark, New Jersey to Tel Aviv on Tuesday and flights from San Francisco and Newark on Wednesday.

"We have issued a travel waiver to allow customers to adjust their travel through May 25 and will continue to monitor the situation," a United spokeswoman said.

A spokesman for Delta, which flies to Tel Aviv from New York's John F. Kennedy International (JFK) airport, said it has issued a travel wavier for flights between Tuesday and Thursday and has "no determination about resumption at this juncture".

American Airlines, which also flies between Tel Aviv and JFK, said it had rebooked customers due to fly on Wednesday and Thursday on alternative flights, without providing details, and had issued a waiver for travel scheduled through May 25.

El Al Israel Airlines said earlier on Wednesday it stood ready to operate extra flights as required "to keep Israel's skies open." This could include special flights to repatriate Israelis stranded "in locations from which it is not possible to return home," it said in a statement.

Israel on Monday briefly halted takeoffs from its main Ben Gurion Airport and diverted incoming flights after a Hamas rocket barrage on the Tel Aviv area. The country has a smaller airport in the South, called Ramon, designed to serve as a stop-gap should wartime threats force longer closures of Ben Gurion. (Reporting by Tracy Rucinski in Chicago; additional reporting by Dan Williams in Jerusalem; Editing by Franklin Paul and Cynthia Osterman)