US President Barack Obama's national intelligence director has resigned after a 16-month tenure¹ marked by a series of security failures.

美国总统奥巴马任下国家情报局长丹尼斯·布莱尔因近期一系列安全问题提出辞职，结束为期16月的任期。

Adm Blair was tasked with co-ordinating 16 US intelligence agencies

Adm Dennis Blair said he had informed the president of his resignation, which goes into effect（生效） on Friday.

His term of office saw the Fort Hood² shooting, the Christmas Day bomb plot and the Times Square car bomb plot.

President Obama said on Thursday he was grateful for Adm Blair's leadership and his "remarkable³ record of service".

"During his time as DNI（国家情报局） , our intelligence community has performed admirably and effectively at a time of great challenges to our security, and I have valued his sense of purpose and patriotism⁴（爱国精神） ," Mr Obama said in a statement.

'Systemic failures'

Adm Blair had drawn⁵ fire for the Christmas Day plot when he said the special interrogation（审问） teams that had been promised were not yet formed.

This was despite a decision to form the teams months earlier.

The final straw may have been the report from the Senate Intelligence Committee, which came out this week, citing "systemic failures" across US intelligence.

"It is with deep regret that I informed the president today that I will step down as director of national intelligence effective Friday," a statement issued by his office said.

"Several strong candidates" were already being interviewed to replace him, officials told the BBC.

Adm Blair is a retired⁶ US Navy admiral（海军上将） who took over as the third director of national intelligence in January 2009.

The position was created after the 11 September 2001 attacks to try to unify⁷ and strengthen America's many intelligence agencies, and to force them to share information.

But Adm Blair's resignation suggests those intelligence reforms have yet to fully⁸ succeed, says the BBC's Adam Brookes in Washington.

Some Republicans defended Adm Blair's record in office.

"It must have been challenging to be forced on the sidelines... but sill catch all the blame for failings," Senator Kit⁹ Bond was quoted as saying by Agence France-Presse.

There is no word yet as to who might replace Mr Blair, our correspondent says.