WASHINGTON (REUTERS) - The United States signed an agreement with Qatar on Monday to sell the Gulf Arab ally Apache attack helicopters and Patriot and Javelin air-defence systems valued at US$11 billion (S$13.6 billion).

"Today's signing ceremony underscores the strong partnership between the United States and Qatar in the area of security and defence and will help improve our bilateral cooperation across a range of military operations," Pentagon spokesman Rear Admiral John Kirby said in a statement.

The agreement was signed at the Pentagon by US Defence Secretary Chuck Hagel and Qatar's defence minister, Hamad bin Ali al-Attiyah.

It was the biggest US arms sale so far this year, according to media reports.

Hagel visited Qatar in December when he and al-Attiyah signed a 10-year Defence Cooperation Agreement to govern interaction between US and Qatari forces and enable the continued assignment of American troops to installations in the area, including the Combined Air Operations Centre at Al Udeid Air Base.

"This is a critically important relationship in the region,"said Kirby. "And the secretary is pleased to be able to continue to make it stronger." Qatar is holding for a year five Taleban prisoners released from the US military prison at Guantanamo Bay, Cuba, in exchange for US Sergeant Bowe Bergdahl, who had been held captive in Afghanistan.