By Humeyra Pamuk, Patricia Zengerle and Mike Stone

WASHINGTON (Reuters) -U.S. President Joe Biden sent a letter to leaders of key Capitol Hill committees on Wednesday informing them of his intention to begin the formal notification process for the sale of (NYSE:) F-16 aircraft to Turkey once Ankara completes Sweden’s NATO accession process.In the letter to the top Republican and Democratic members of the Senate Foreign Relations and House of Representatives Foreign Affairs committees, Biden urged Congress to approve the sale "without delay," a U.S. official said. Earlier on Wednesday the White House sent a letter to members of Congress urging approval of the $20 billion sale of F-16 aircraft and modernization kits to Turkey, four sources familiar with the letter told Reuters.

Turkey's parliament ratified Sweden's NATO membership bid on Tuesday, clearing a major hurdle to expanding the Western military alliance after 20 months of delay. The sources said the letter was sent on Wednesday, and that the Biden administration has not yet formally notified Congress of plans for the sale.

Turkey's delay in approving the ratification had been a major obstacle to winning congressional approval for the fighter jet deal. Lawmakers had said they were awaiting Turkey's approval of Sweden's NATO membership- including President Tayyip Erdogan's signature - before deciding whether to approve the sale.

The White House did not respond to a request for comment.

The U.S. State Department also urged Ankara on Wednesday to formally finalize Sweden's NATO ratification. To do that, Erdogan needs to sign the legislation, which then would be published in Turkey's Official Gazette. The instrument of accession for Sweden also needs to be sent to Washington.

The State Department declined to provide an exact timeline on the formal notification process for the F-16 sale.

"President Biden, Secretary Blinken have been very clear of our support for modernizing Turkey's F-16 fleet, which we view as a key investment in NATO interoperability. But beyond that ... I'm just not going to confirm or get ahead of proposed defense sales or transfers until they are formally notified to Congress," State Department Deputy Spokesperson Vedant Patel told a news briefing, referring to Secretary of State Antony Blinken.

Turkey in October 2021 asked to purchase $20 billion of Lockheed Martin F-16 fighters and nearly 80 modernization kits for its existing warplanes.

Leaders of the U.S. Senate Foreign Relations and House Foreign Affairs committees review every major foreign arms sale. They regularly ask questions or raise concerns over human rights or diplomatic issues that can delay or stop such deals.Senator Chris Van Hollen, a Democrat who sits on the Senate Foreign Relations Committee, cast some doubt on a speedy approval, saying lawmakers need assurances from the Biden administration and Turkey first.

“For much of the time President Erdogan has been in office, Turkey has been an unfaithful NATO ally — so this is welcome news," Van Hollen said.

"That said, I still have questions about Erdogan’s ongoing attacks against our Syrian Kurdish allies, his aggressive actions in the Eastern Mediterranean, and the role he played in supporting Azerbaijan’s military assaults against Nagorno-Karabakh," Van Hollen told Reuters.

Sweden and Finland applied to enter NATO after Russia invaded Ukraine in February 2022. While Finnish membership was sealed last year, Sweden's bid had been held up by Turkey and Hungary.

All NATO members need to approve applications from countries seeking to join the alliance. When Sweden and Finland asked to join, Turkey raised objections over what it said was the two countries' protection of groups it deems terrorists.