NEW YORK — American Airlines and JetBlue Airways have formed a partnership to offer passengers connections to flights in and out of New York and Boston.

JetBlue customers will be able to connect on flights to 12 of American’s international destinations from JFK and Boston. American customers can book nonstop JetBlue flights from JFK and Boston to 18 domestic spots that American doesn’t serve from those cities.

The deal will feed new passengers into both carriers’ networks by allowing them to book both legs of their flight at the same time. For example, it will provide streamlined service for customers who fly from Nantucket, Mass., to JFK on JetBlue and from there to London on American.

American, a unit of AMR Corp., also plans to add 23 more flights to and from seven new destinations out of New York’s JFK and LaGuardia airports by the end of the year.

Combined with earlier announcements, American and sister carrier American Eagle will add 31 total flights and 13 additional routes to and from the two airports by the end of 2010. The first few flights begin in April. In a news conference about the partnership, American CEO Gerard Arpey said he doesn’t expect the new flights to cause major problems at JFK, where the main runway is shutdown until June 30 for repairs.

Arpey said the carrier timed most of the flights to launch after the runway’s scheduled reopening. American has a thinner flight schedule during the construction to ease congestion. JetBlue and Delta, the two largest carriers at the airport, also have lighter schedules during the period.

American, based in Fort Worth, Texas, has also pledged more than $30 million for terminal improvements at the two airports.

JetBlue CEO Dave Barger said at the news conference that the runway repairs are “going very, very well” and said construction might end ahead of schedule.

JetBlue also said it plans to launch service at Reagan National in November. The airline, based in Forest Hills, N.Y., said it will initially offer at least eight daily departures from Reagan to East Coast cities.

The carrier already operates at the two other Washington-area airports, Dulles International Airport and Baltimore/Washington International.

The new flights are a result of so-called “slot swaps” that were part of the agreement between the two carriers. A slot is an interval of time during which an airline can takeoff or land at an airport.

American will transfer eight slot pairs at Ronald Reagan National Airport and one slot pair at Westchester Airport in White Plains, N.Y. to JetBlue. In exchange, JetBlue will transfer 12 slot pairs at JFK to American.

Along with the eight daily departures at Reagan, JetBlue has also asked the Department of Transportation for approval to get slots from US Airways that would give JetBlue five more takeoff and landing times at that airport.