LOT Polish Airlines and Turkish Airlines have signed an agreement extending their strategic partnership to offer "more connection alternatives and even better product".
The Star Alliance carriers first signed a codeshare deal covering flights between Warsaw and Istanbul in 2000.
This was extended last year to include Istanbul routes to/from Amman, Cairo and Anakara; and to include Warsaw routes to/from Vienna and Krakow.
The exact details of the latest expansion have not been announced.
A statement said: "Final model of cooperation has not been decided yet, however both airlines agreed to proceed towards future joint venture cooperation."
LOT Polish CEO Sebastian Mikosz said: "LOT and Turkish Airlines have been partners for many years. I am convinced that this letter of understanding will help us to create unique and global offer.
"This will give our passengers all advantages of worldwide and supplementary networks of both carriers."
Temel Kotil, CEO of Turkish Airlines, added: "This strategic partnership between our airlines will enable both carriers to enhance their presence on their respective markets, while contributing their efficiency on the routes agreed within the scope of extended cooperation.
In June, LOT Polish offered a first look at the new business, premium economy and economy class seating to be installed on its B787 Dreamliner aircraft (see news, June 29).